LintCode 32. 最小子串覆蓋

給定一個字符串source和一個目標字符串target，在字符串source中找到包括所有目標字符串字母的子串。

public class Solution {
    /**
     * @param source : A string
     * @param target: A string
     * @return: A string denote the minimum window, return "" if there is no such a string
     */
    public String minWindow(String source , String target) {
        // write your code here
        int[] map = new int[128];
        for (int i = 0; i < map.length; i++) {
            map[i] = 0;
        }
        for (int i = 0; i < target.length(); i++) {
            map[target.charAt(i)]++;
        }
        int counter = target.length(), begin = 0, end = 0, d = Integer.MAX_VALUE, head = 0;
        while (end < source.length()) {
            if (map[source.charAt(end++)]-- > 0) {
                counter--;
            }
            while (counter == 0) {
                if (end - begin < d) {
                    d = end - begin;
                    head = begin;
                }
                if (map[source.charAt(begin++)]++ == 0) {
                    counter++;
                }
            }
        }
        return d == Integer.MAX_VALUE ? "" : source.substring(head, head + d);
    }
}