76. 最小覆盖子串

滑动窗口

经典写法

class Solution {public String minWindow(String s, String t) {HashMap<Character, Integer> window = new HashMap<>(), need = new HashMap<>();for(char c : t.toCharArray()) need.merge(c, 1, Integer::sum);int cnt = 0, start = 0, len = Integer.MAX_VALUE;for(int l = 0, r = 0; r < s.length(); r++){// 窗口增大的逻辑char c = s.charAt(r);if(need.containsKey(c)){window.merge(c, 1, Integer::sum);if(need.get(c).equals(window.get(c))) cnt++;}while(cnt == need.size()){  // 窗口何时缩小// 更新答案if(len > r - l + 1){len = r - l + 1;start = l;}// 窗口缩小的逻辑c = s.charAt(l++);if(need.containsKey(c)){if(need.get(c).equals(window.get(c))) cnt--;window.merge(c, -1, Integer::sum);}}}return len == Integer.MAX_VALUE ? "" : s.substring(start, start + len);}
}

简洁写法

class Solution {public String minWindow(String s, String t) {char[] chs = s.toCharArray(), cht = t.toCharArray();int[] hash = new int[128];for(char c : cht) hash[c]--;int cnt = 0, m = cht.length;String res = "";for(int l = 0, r = 0; r < chs.length; r++){hash[chs[r]]++;if(hash[chs[r]] <= 0) cnt++;while(cnt == m && hash[chs[l]] > 0) hash[chs[l++]]--;if(cnt == m){if(res.equals("") || res.length() > r - l + 1){res = s.substring(l, r + 1);}}}return res;}
}

一个坑

两个 Integer 相等比较应该用 equals，而不是 ==。
一个 Integer 和一个 int 比较，Integer 会自动拆箱，可以用 ==。

class Solution {public String minWindow(String s, String t) {int i = 0;HashMap<Character, Integer> mapS = new HashMap<>();HashMap<Character, Integer> mapT = new HashMap<>();for(char c : t.toCharArray()){//mapT.merge(c, 1, Integer::sum);mapT.put(c, mapT.getOrDefault(c, 0) + 1);}int cnt = 0;int res = Integer.MAX_VALUE, left = 0, right = 0;for(int j = 0; j < s.length(); j++){char c = s.charAt(j);if(mapT.containsKey(c)){//mapS.merge(c, 1, Integer::sum);mapS.put(c, mapS.getOrDefault(c, 0) + 1);if(mapS.get(c).equals(mapT.get(c))){  // 一个坑：Integer 相等比较应该用 equals，而不是 ==cnt++;}}while(cnt == mapT.size()){if(res > j - i + 1){left = i;right = j;res = j - i + 1;}c = s.charAt(i++);if(mapT.containsKey(c)){//mapS.merge(c, -1, Integer::sum);mapS.put(c, mapS.getOrDefault(c, 0) - 1);if(mapS.get(c).compareTo(mapT.get(c)) < 0){cnt--;}}}}return res == Integer.MAX_VALUE ? "" : s.substring(left, right + 1);}
}