题目：

题解：

char* removeDuplicateLetters(char* s) {     int vis[26], num[26];     memset(vis, 0, sizeof(vis));     memset(num, 0, sizeof(num));      int n = strlen(s);     for (int i = 0; i < n; i++) {         num[s[i] - 'a']++;     }      char* stk = malloc(sizeof(char) * 27);     int stkTop = 0;     for (int i = 0; i < n; i++) {         if (!vis[s[i] - 'a']) {             while (stkTop > 0 && stk[stkTop - 1] > s[i]) {                 if (num[stk[stkTop - 1] - 'a'] > 0) {                     vis[stk[--stkTop] - 'a'] = 0;                 } else {                     break;                 }             }             vis[s[i] - 'a'] = 1;             stk[stkTop++] = s[i];         }         num[s[i] - 'a'] -= 1;     }     stk[stkTop] = '\0';     return stk; }