0035_P76_Minimum_Window_Substring.cpp

/* 
Day: 35
Problem Number: 76 (https://leetcode.com/problems/minimum-window-substring)
Date: 04-02-2024 
Description:
Given two strings s and t of lengths m and n respectively, return the minimum window 
substring of s such that every character in t (including duplicates) is included in the window. If there is no such substring, return the empty string "".

The testcases will be generated such that the answer is unique.

Example 1:
Input: s = "ADOBECODEBANC", t = "ABC"
Output: "BANC"
Explanation: The minimum window substring "BANC" includes 'A', 'B', and 'C' from string t.

Example 2:
Input: s = "a", t = "a"
Output: "a"
Explanation: The entire string s is the minimum window.

Example 3:
Input: s = "a", t = "aa"
Output: ""
Explanation: Both 'a's from t must be included in the window.
Since the largest window of s only has one 'a', return empty string.

Constraints:
* m == s.length
* n == t.length
* 1 <= m, n <= 105
* s and t consist of uppercase and lowercase English letters.
 
Follow up: Could you find an algorithm that runs in O(m + n) time?
Code:  */
class Solution {
public:
    string minWindow(string s, string t) {
        int need[128]{};
        int window[128]{};
        int m = s.size(), n = t.size();
        for (char& c : t) {
            ++need[c];
        }
        int cnt = 0, j = 0, k = -1, mi = 1 << 30;
        for (int i = 0; i < m; ++i) {
            ++window[s[i]];
            if (need[s[i]] >= window[s[i]]) {
                ++cnt;
            }
            while (cnt == n) {
                if (i - j + 1 < mi) {
                    mi = i - j + 1;
                    k = j;
                }
                if (need[s[j]] >= window[s[j]]) {
                    --cnt;
                }
                --window[s[j++]];
            }
        }
        return k < 0 ? "" : s.substr(k, mi);
    }
};