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public class Solution { public List<Integer> slidingWindowTemplateByHarryChaoyangHe(String s, String t) { //init a collection or int value to save the result according the question. List<Integer> result = new LinkedList<>(); if(t.length()> s.length()) return result; //create a hashmap to save the Characters of the target substring. //(K, V) = (Character, Frequence of the Characters) Map<Character, Integer> map = new HashMap<>(); for(char c : t.toCharArray()){ map.put(c, map.getOrDefault(c, 0) + 1); } //maintain a counter to check whether match the target string. int counter = map.size();//must be the map size, NOT the string size because the char may be duplicate. //Two Pointers: begin - left pointer of the window; end - right pointer of the window int begin = 0, end = 0; //the length of the substring which match the target string. int len = Integer.MAX_VALUE; //loop at the begining of the source string while(end < s.length()){ char c = s.charAt(end);//get a character if( map.containsKey(c) ){ map.put(c, map.get(c)-1);// plus or minus one if(map.get(c) == 0) counter--;//modify the counter according the requirement(different condition). } end++; //increase begin pointer to make it invalid/valid again while(counter == 0 / counter condition. different question may have different condition /){ char tempc = s.charAt(begin);//**be careful here: choose the char at begin pointer, NOT the end pointer if(map.containsKey(tempc)){ map.put(tempc, map.get(tempc) + 1);//plus or minus one if(map.get(tempc) > 0) counter++;//modify the counter according the requirement(different condition). } / save / update(min/max) the result if find a target*/ // result collections or result int value begin++; } } return result; } }