76. Minimum Window Substring

Given a string S and a string T, find the minimum window in S which will contain all the characters in T in complexity O(n).

For example, S = "ADOBECODEBANC" T = "ABC" Minimum window is "BANC".

Note: If there is no such window in S that covers all characters in T, return the empty string "".

If there are multiple such windows, you are guaranteed that there will always be only one unique minimum window in S.

needs two pointers, the front pointer make sure to include all the chars in T, the end pointer make sure to exclude as many chars as possible to make the window smallest.

public class Solution {
    // you need to include the 
    public String minWindow(String s, String t) {

        Map<Character, Integer> target = new HashMap<>();
        Map<Character, Integer> found = new HashMap<>();

        int minWindowStart = -1;
        int minWindowLen = Integer.MAX_VALUE;

        for(char ch : t.toCharArray()){
            int count   =1;
                count = target.get(ch) + 1;
            target.put(ch, count);

        int start = 0;
        int foundNumber = 0;// this means all letter shows up, you need to shrink window when possible.
        for(int i=0; i<s.length(); i++){
            // move i to include all chars in target.
            char c = s.charAt(i);
            if(!target.containsKey(c)) continue;

            int k = 1;
                k = found.get(c) + 1;
            found.put(c, k);
            //once foundNumber reach t.length(), the number stands. found.get(c) will be updated in the next block
            if(found.get(c) <= target.get(c)){

            if(foundNumber == t.length()){ // you don't decrease foundNumber after you found require the number of chars.
                char begin = s.charAt(start);
                // move start as right as possbile, to shrink the window size.
                while(!found.containsKey(begin) || found.get(begin) > target.get(begin)){

                        found.put(begin, found.get(begin)-1);
                        //you can NOT decrease foundNumber here. there is no go back for foundNumber.
                    begin = s.charAt(start);

                int len = i-start+1;
                if(len < minWindowLen){
                    minWindowLen = len;
                    minWindowStart = start;

        if(minWindowStart != -1){
            return s.substring(minWindowStart, minWindowStart + minWindowLen);
        return "";

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