828. Unique Letter String(#1 ).java

class Solution {
	    public int uniqueLetterString(String S) {
	        char[] s = S.toCharArray();
	        int n = s.length;
	        int mod = 1000000007;
	        long ret = 0;
	        for(int i = 0;i < s.length;i++){
	        	int l = i-1;
	        	for(;l >= 0 && s[l] != s[i];l--);
	        	int r = i+1;
	        	for(;r < n && s[r] != s[i];r++);
	        	ret += (long)(r-i)*(i-l);
	        }
	        return (int)(ret%mod);
	    }
	}