代码1

Runtime: 3 ms, faster than 97.65% of Java online submissions for Minimum Window Substring.

class Solution {
    public String minWindow(String s, String t) {
        int[] source = new int[256];
        int[] target = new int[256];
        char[] s_char = s.toCharArray();
        char[] t_char = t.toCharArray();
        for (int i = 0; i < t_char.length; i++) {
            target[t_char[i]]++;
        }
        String ans = "";
        int length = s_char.length;
        int found = 0;
        for (int i = 0, j = 0; j < s_char.length; j++) {
            source[s_char[j]]++;
            if (source[s_char[j]] <= target[s_char[j]]) found++;
            if (found == t_char.length) {
                
                while (i < s_char.length 
                       && source[s_char[i]] > target[s_char[i]])
                    //排除开头多余的字符
                    source[s_char[i++]]--;
                //如果长度有变小就保存
                if (j - i + 1 <= length) {
                    ans = s.substring(i, j + 1);
                    length = j - i + 1;
                }
                //把第一个有用的字符排除看看之后有没有更小的
                source[s_char[i++]]--;
                found--;
            }          
        }
        return ans;
    }
}