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).

Example:

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Input: S = "ADOBECODEBANC", T = "ABC"
Output: "BANC"

Note:

  • If there is no such window in S that covers all characters in T, return the empty string “”.
  • If there is such window, you are guaranteed that there will always be only one unique minimum window in S.

Solution:

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class Solution {
    public String minWindow(String s, String t) {
        int[] freq = new int[128];
        int counter = t.length();
        for (char c : t.toCharArray()) {
            freq[c]++;
        }
        int start = 0;
        int end = 0;
        int minStart = 0;
        int minLength = Integer.MAX_VALUE;

        // Move end to find a valid window.
        while (end < s.length()) {
            char c = s.charAt(end);

            if (freq[c] > 0) {
                counter--;
            }
            freq[c]--;
            end++;
            while (counter == 0) {
                if (end - start < minLength) {
                    minStart = start;
                    minLength = end - start;
                }

                char startChar = s.charAt(start);
                freq[startChar]++;

                if (freq[startChar] > 0) {
                    counter++;
                }
                start++;
            }
        }
        if (minLength == Integer.MAX_VALUE) {
            return "";
        }

        return s.substring(minStart, minStart + minLength);
    }
}