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Commitacf5d3f

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Create Combinations.java
Problem statement: Given two integers n and k, return all possible combinations of k numbers out of 1 ... n. For example, If n = 4 and k = 2, a solution is: [[2,4], [3,4], [2,3], [1,2], [1,3], [1,4]]Strategy: Determin the starting number `start` among all k numbers as a first step. Afterwards, repeat the same problem by changing k to k-1 and changing possible starting number from `start` to `start+1`
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‎DFS/Combinations.java

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/*
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Problem statement: Given two integers n and k, return all possible combinations of k numbers out of 1 ... n. For example, If n = 4 and k = 2, a solution is: [[2,4], [3,4], [2,3], [1,2], [1,3], [1,4]]
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Strategy: Determin the starting number `start` among all k numbers as a first step. Afterwards, repeat the same problem by changing k to k-1 and changing possible starting number from `start` to `start+1`
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*/
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publicList<List<Integer>>combine(intn,intk) {
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List<List<Integer>>res =newLinkedList<>();
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if(k>n)returnres;
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helper(n,k,1,newLinkedList<Integer>(),res);
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returnres;
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}
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privatevoidhelper(intn,intk,intstart,List<Integer>list,List<List<Integer>>res) {
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if(k==1) {
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for(inti=start;i<=n;i++) {
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List<Integer>inner =newLinkedList<Integer>(list);
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inner.add(i);
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res.add(inner);
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}
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return;
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}
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for(inti=start;i<=n-k+1;i++) {
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List<Integer>inner =newLinkedList<Integer>(list);
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inner.add(i);
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helper(n,k-1,i+1,inner,res);
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}
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}

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