76-Minimum-Window-Substring.cpp

/*
    Given 2 strings s & t, return min window substring
    of s such that all chars in t are included in window
    Ex. s = "ADOBECODEBANC" t = "ABC" -> "BANC"

    Sliding window + hash map {char -> count}
    Move j until valid, move i to find smaller

    Time: O(m + n)
    Space: O(m + n)
*/

class Solution {
public:
    string minWindow(string s, string t) {
        // count of char in t
        unordered_map<char, int> m;
        for (int i = 0; i < t.size(); i++) {
            m[t[i]]++;
        }
        
        int i = 0;
        int j = 0;
        
        // # of chars in t that must be in s
        int counter = t.size();
        
        int minStart = 0;
        int minLength = INT_MAX;
        
        while (j < s.size()) {
            // if char in s exists in t, decrease
            if (m[s[j]] > 0) {
                counter--;
            }
            // if char doesn't exist in t, will be -'ve
            m[s[j]]--;
            // move j to find valid window
            j++;
            
            // when window found, move i to find smaller
            while (counter == 0) {
                if (j - i < minLength) {
                    minStart = i;
                    minLength = j - i;
                }
                
                m[s[i]]++;
                // when char exists in t, increase
                if (m[s[i]] > 0) {
                    counter++;
                }
                i++;
            }
        }
        
        if (minLength != INT_MAX) {
            return s.substr(minStart, minLength);
        }
        return "";
    }
};