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Commitbde1f4e

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7, 28, 125, 204, 228, 278
1 parent350acd9 commitbde1f4e

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7 files changed

+173
-1
lines changed

7 files changed

+173
-1
lines changed

‎Easy/125-validPalindrome.js‎

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/**
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* Key: use regular expression to find only alpha and numeric letters.
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* Two pointers: start and end
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*
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*@param {string} s
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*@return {boolean}
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*/
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varisPalindrome=function(s){
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varpattern=/[a-z0-9]/gi;
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s=s.match(pattern);
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if(!s||s.length===1)returntrue;
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for(vari=0;i<s.length/2;i++){
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if(s[i].toLowerCase()!==s[s.length-1-i].toLowerCase())returnfalse;
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}
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returntrue;
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};

‎Easy/204-countPrimes.js‎

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/**
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*@param {number} n
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*@return {number}
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*/
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varcountPrimes=function(n){
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varcount=0;
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for(vari=1;i<n;i++){
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if(isPrime(i))count++;
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}
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returncount;
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};
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// traditional approach, to determine a number is prime or not,
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// only need to consider factors up to √n
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varisPrime=function(num){
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if(num<=1)returnfalse;
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// Loop's ending condition is i * i <= num instead of i <= sqrt(num)
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// to avoid repeatedly calling an expensive function sqrt().
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for(vari=2;i*i<=num;i++){
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if(num%i==0)returnfalse;
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}
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returntrue;
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};
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/**
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* A better solution using Sieve of Eratosthenes
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* if the current number is p,
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* mark off multiples of p starting at p^2, then in increments of p: p^2 + p, p^2 + 2p, ...
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* these above numbers are not prime numbers
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*
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*@param {number} n
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*@return {number}
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*/
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varcountPrimes=function(n){
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varcount=0;
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varisPrime=[];
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for(vari=2;i<n;i++)isPrime[i]=true;
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for(vari=2;i*i<n;i++){
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if(isPrime[i]){
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varstart=i*i;
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while(start<=n){
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isPrime[start]=false;
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start=start+i;
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}
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}
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}
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for(varj=2;j<n;j++){
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if(isPrime[j])count++;
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}
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returncount;
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};

‎Easy/228-summaryRanges.js‎

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/**
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*@param {number[]} nums
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*@return {string[]}
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*/
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varsummaryRanges=function(nums){
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varranges=[];
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for(vari=0;i<nums.length;i++){
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varj=i;
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while(nums[j+1]){
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if((nums[j+1]-nums[j])===1)j++;
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elsebreak;
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}
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if(nums[i]!==nums[j]){
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ranges.push(nums[i]+'->'+nums[j]);
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}else{
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ranges.push(''+nums[i]);
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}
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i=j;
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}
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returnranges;
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};

‎Easy/278-firstBadVersion.js‎

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/**
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* Definition for isBadVersion()
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*
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*@param {integer} version number
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*@return {boolean} whether the version is bad
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* isBadVersion = function(version) {
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* ...
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* };
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*/
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/**
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*@param {function} isBadVersion()
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*@return {function}
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*/
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varsolution=function(isBadVersion){
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/**
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* Key: binary search, be careful to set low and high correctly
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*
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*@param {integer} n Total versions
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*@return {integer} The first bad version
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*/
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returnfunction(n){
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varlow=1;
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varhigh=n;
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while(low<high){
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// be careful in JavaScript, using Math.floor to get the integer part
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varmid=Math.floor(low+(high-low)/2);
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if(!isBadVersion(mid))low=mid+1;
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// and not `mid - 1` to high
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elsehigh=mid;
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}
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returnlow;
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};
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};

‎Easy/28-implementstrStr.js‎

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/**
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*@param {string} haystack
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*@param {string} needle
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*@return {number}
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*/
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varstrStr=function(haystack,needle){
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varhIndex=0;
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while((haystack.length-hIndex)>=needle.length){
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varnIndex=0;
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if(haystack[hIndex]===needle[nIndex]){
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while(nIndex<needle.length){
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if(haystack[hIndex+nIndex]!==needle[nIndex])break;
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nIndex++;
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}
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}
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if(nIndex===needle.length)break;
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elsehIndex++;
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}
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if((haystack.length-hIndex)<needle.length)return-1;
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returnhIndex;
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};
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// a better solution? KMP?

‎Easy/303-rangeSumQuery.js‎

Lines changed: 4 additions & 1 deletion
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@@ -1,6 +1,9 @@
11
/**
22
* Key: Dynamic Programming
3-
*
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* add an array to track the sum of previous elements
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* sums[i] = sum[i-1] + nums[i-1];
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* sumRange[i, j] = sums[j+1] - sums[i];
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*
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*@constructor
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*@param {number[]} nums
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*/

‎Easy/7-reverseInteger.js‎

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/**
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*@param {number} x
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*@return {number}
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*/
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varreverse=function(x){
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varrevX=0;
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varabsX=Math.abs(x);
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while(absX>0){
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varlast=absX%10;
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revX=revX*10+last;
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absX=(absX-last)/10;
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}
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// in case overflow, in JavaScript maximum number is 2^31 - 1
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if(revX>Math.pow(2,31)-1)return0;
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if(x<0)revX=-revX;
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returnrevX;
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};

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