Leetcode: Find All Anagrams in a String
Given a string s and a non-empty string p, find all the start indices of p's anagrams in s. Strings consists of lowercase English letters only and the length of both strings s and p will not be larger than 20,100. The order of output does not matter. Example 1: Input: s: "cbaebabacd" p: "abc" Output: [0, 6] Explanation: The substring with start index = 0 is "cba", which is an anagram of "abc". The substring with start index = 6 is "bac", which is an anagram of "abc". Example 2: Input: s: "abab" p: "ab" Output: [0, 1, 2] Explanation: The substring with start index = 0 is "ab", which is an anagram of "ab". The substring with start index = 1 is "ba", which is an anagram of "ab". The substring with start index = 2 is "ab", which is an anagram of "ab".
Time Complexity will be O(n) because the "start" and "end" points will only move from left to right once.
Sliding Window: Use a count to denote the difference between current sliding window and p, if count == 0, means the current sliding window is the same with p, add start to the result
refer to https://discuss.leetcode.com/topic/64434/shortest-concise-java-o-n-sliding-window-solution/2
1 public class Solution { 2 public List<Integer> findAnagrams(String s, String p) { 3 List<Integer> list = new ArrayList<>(); 4 if (s == null || s.length() == 0 || p == null || p.length() == 0) return list; 5 6 int[] hash = new int[256]; //character hash 7 8 //record each character in p to hash 9 for (char c : p.toCharArray()) { 10 hash[c]++; 11 } 12 //two points, initialize count to p's length 13 int left = 0, right = 0, count = p.length(); 14 15 while (right < s.length()) { 16 //move right everytime, if the character exists in p's hash, decrease the count 17 //current hash value >= 1 means the character is existing in p 18 if (hash[s.charAt(right)] >= 1) { //char at right is in p, so is needed to make an anagram of p, so if we shift our window to 19 count--; // include right, the difference between our window and p will decrease by 1, so count-1 20 } 21 hash[s.charAt(right)]--; //Other case if hash[s.charAt(right)] right now == 0, which means we do not need it, we do not decrease count 22 right++; // hash[s.charAt(right)] will be -1, if window contains some char that p do not need, count will never reach 0 23 // whereas some hash[somewhere] will be negative, some hash[somewhereelse] will be positive 24 //when the count is down to 0, means we found the right anagram 25 //then add window's left to result list 26 if (count == 0) { // this is only when p's chars and window's chars are all the same 27 list.add(left); 28 } 29 //if we find the window's size equals to p, then we have to move left (narrow the window) to find the new match window 30 //++ to reset the hash because we kicked out the left 31 //only increase the count if the character is in p 32 //the count >= 0 indicate it was original in the hash, cuz it won't go below 0 33 if (right - left == p.length() ) { 34 35 if (hash[s.charAt(left)] >= 0) { 36 count++; 37 } 38 hash[s.charAt(left)]++; 39 left++; 40 41 } 42 43 44 } 45 return list; 46 } 47 }