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;
}