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int findSubstring(String s) {
    int[] map = new int[256];
    int counter; // check whether the substring is valid
    int begin = 0, end = 0; //two pointers, one point to tail and one  head
    int d; //the length of substring
    for () { /* initialize the hash map here */ }
    while (end < s.length()) {
        if (map[s[end++]]-- ?) {  /* modify counter here */ }
        while (/* counter condition */) {
             /* update d here if finding minimum*/
            //increase begin to make it invalid/valid again
            if (map[s[begin++]]++ ?) { /*modify counter here*/ }
        }
        /* update d here if finding maximum*/
    }
    return d;
}

Related Problems:

Ref: https://discuss.leetcode.com/topic/30941/here-is-a-10-line-template-that-can-solve-most-substring-problems