Minimum Window Substring

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

Time Complexity: $O(n)$, Space Complexity: $O(m)$

class Solution {
public:
    string minWindow(string s, string t) {
        unordered_map<char, int> mp;
        for(auto c: t) {
            mp[c]++;
        }
 
        int j = 0, n = s.length(), count = mp.size(), window = INT_MAX, start = -1;
        for(int i = 0; i < n; i++) {
            while(j < n && count > 0) {
                if(mp.find(s[j]) != mp.end()) {
                    mp[s[j]]--;
                    if(mp[s[j]] == 0) 
                        count--;
                }
                j++;
            }
 
            if(count == 0) {
                if(j - i < window) {
                    window = j - i;
                    start = i;
                }
            }
                
            
            if(mp.find(s[i]) != mp.end()) {
                mp[s[i]]++;
                if(mp[s[i]] == 1)
                    count++;
            }
        }
 
        return window == INT_MAX ? "" : s.substr(start, window);
    }
};

Source

• Minimum Window Substring - LeetCode