leetcode — minimum-window-substring

import java.util.HashMap;
import java.util.Map;

/**
*
* Source : https://oj.leetcode.com/problems/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 emtpy string "".
*
* > If there are multiple such windows, you are guaranteed that there
*   will always be only one unique minimum window in S.
*
*/
public class MinimumWindowSubstring {

/**
* 寻找S中包含T的最短字符串
*
* 窗口的方法，
* 先将T中所有字符保存在hash表中，值为每个字符出现的次数
* 从头开始遍历，判断该字符是否在hash表中，如果在，则将该字符对应的值减1，并记录此时已包含T中字符总数
* 如果字符总数等于T的长度说明找到一个子串，找到第一个包含T的字符串，然后将此时子串的长度和之前最小长度比较，更新最小字串的长度，并记录此时子串的起始位置
* 然后移动子串的左边界，略过不再T中的字符
* 当遍历完成的时候，记录的最小字串的起始位置和最小字串的长度可以得到包含T的最小字串
*
* @param S
* @param T
* @return
*/
public String findMinimumWindowSubString (String S, String T) {
Map<Character, Integer> map = new HashMap<Character, Integer>();
// 初始化hash表
for (int i = 0; i < T.length(); i++) {
if (map.keySet().contains(T.charAt(i))) {
map.put(T.charAt(i), map.get(T.charAt(i)) + 1);
} else {
map.put(T.charAt(i), 1);
}
}

int minLen = S.length();
int left = 0;
int minStart = 0;
int count = 0;
for (int i = 0; i < S.length(); i++) {
Character ch = S.charAt(i);
if (map.keySet().contains(ch)) {
map.put(ch, map.get(ch) - 1);
if (map.get(ch) >= 0) {
count ++;
}
// 如果找到一个子串
while (count == T.length()) {
if (i - left + 1 < minLen) {
minLen = i - left + 1;
minStart = left;
}
if (map.keySet().contains(S.charAt(left))) {
// 如果右移的时候遇到了T中的字符，将hash表中对应字符加1，因为在后面要查找该字符
map.put(S.charAt(left), map.get(S.charAt(left)) + 1);
if (map.get(S.charAt(left)) > 0) {
// 如果加1之后小于1，说明之前是负数，说明现在窗口内还有多个T中的字符，需要继续向前移动窗口
// 如果大于1才停止向前移动窗口
count--;
}
}
left ++;
}
}
}
if (minLen > S.length()) {
return "";
}
return S.substring(minStart, minLen + minStart);

}

public static void main(String[] args) {
MinimumWindowSubstring minimumWindowSubstring = new MinimumWindowSubstring();
System.out.println(minimumWindowSubstring.findMinimumWindowSubString("ADOBECODEBANC", "ABC"));
}
}