LeetCode #3 Longest Substring Wi

/**
* This is an improved version over the previous submission: https://www.jianshu.com/p/56eac9b1f73c
*/

void clear(int *a, char *s, int start, int end) { for (int i = start; i <= end; i++) { a[s[i]] = -1; } }

#define R 128//capacity of the character set
int lengthOfLongestSubstring(char * s) {
    if (s == NULL || *s == '\0') return 0;
    int n = strlen(s);
    int *a = (int*)malloc(R * sizeof(*a));
    for (int i = 0; i < R; i++) { a[i] = -1; }
    int len = 0, start = 0;
    for (int i = 0; i < n; i++) {
        if (a[s[i]] != -1) {
            int t = i - start;
            if (len < t) { len = t; }
            start = a[s[i]] + 1;
            clear(a, s, i - t/*i - t = start*/, a[s[i]]);
        }
        a[s[i]] = i;
    }
    int t = n - start;
    if (len < t) { len = t; }
    return len;
}