316. Remove Duplicate Letters

Problem

Tags: String, Stack, Greedy, Monotonic Stack

Given a string s, remove duplicate letters so that every letter appears once and only once. You must make sure your result is the smallest in lexicographical order among all possible results.

Example 1:

Input: s = "bcabc"
Output: "abc"

Example 2:

Input: s = "cbacdcbc"
Output: "acdb"

Constraints:

• 1 <= s.length <= 10^4
• s consists of lowercase English letters.

Note: This question is the same as 1081: https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/

Code

C

// 316. Remove Duplicate Letters (9/10/54180)
// Runtime: 0 ms (94.51%) Memory: 5.68 MB (90.11%) 

char* removeDuplicateLetters (char* s) {
    int last_pos[26] = { -1 };
    for (int i = 0; s[i] != '\0'; i++) {
        last_pos[s[i] - 'a'] = i;
    }
    
    bool used[26] = { false };
    int stk[10001] = { 0 };
    int stk_size = 0;
    for (int i = 0; s[i] != '\0'; i++) {
        int code = s[i] - 'a';
        if (used[code]) {
            continue;
        }
        
        while (stk_size) {
            int prev = stk[stk_size - 1];
            if (prev < code || i >= last_pos[prev]) {
                break;
            }
            stk_size--;
            used[prev] = false;
        }
        
        stk[stk_size++] = code;
        used[code] = true;
    }
    
    char* ans = (char*)calloc(stk_size + 1, sizeof(char));
    for (int i = 0; i < stk_size; i++) {
        ans[i] = stk[i] + 'a';
    }
    
    return ans;
}