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Merge pull requestneetcode-gh#2199 from seinlin/1930

Add 1930-unique-length-3-palindromic-subsequences.c

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	`1`	`+intcountPalindromicSubsequence(char*s){`
	`2`	`+intn=strlen(s);`
	`3`	`+inti,j;`
	`4`	`+intc,answer;`
	`5`	`+intfirst[26]= {[0 ...25]=-1};`
	`6`	`+intlast[26]= {[0 ...25]=-1};`
	`7`	`+`
	`8`	`+// Use two arrays to record the first and last`
	`9`	`+// location of each character.`
	`10`	`+for (i=0;i<n;i++) {`
	`11`	`+c=s[i]-'a';`
	`12`	`+if (first[c]==-1) {`
	`13`	`+first[c]=i;`
	`14`	`+ }else {`
	`15`	`+last[c]=i;`
	`16`	`+ }`
	`17`	`+ }`
	`18`	`+`
	`19`	`+// When a character appears twice, count all the`
	`20`	`+// unique character between them.`
	`21`	`+answer=0;`
	`22`	`+for (i=0;i<26;i++) {`
	`23`	`+if (last[i]!=-1) {`
	`24`	`+intm[26]= {[0 ...25]=-1};`
	`25`	`+for (j=first[i]+1;j<last[i];j++) {`
	`26`	`+intc=s[j]-'a';`
	`27`	`+if (m[c]==-1) {`
	`28`	`+m[c]=j;`
	`29`	`+answer++;`
	`30`	`+ }`
	`31`	`+ }`
	`32`	`+ }`
	`33`	`+ }`
	`34`	`+`
	`35`	`+returnanswer;`
	`36`	`+}`

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