LeetCode - The Sliding Window Part

This part is Sliding Window related problems.

The sliding window problem is a common type of problems. The main idea is use two pointers. One points to the start position, one points to the end position. And use some data structure to store the context from start to end, normally it’s a HashMap. And check whether it’s meet the conditions. And the key of this type of problems is when to move the start pointer and the end pointer.

Here’s the template of this type of problems.

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;
    }
}

The idea is to move end pointer each step, and check the conditions. If the conditions are meet, we can move the start pointer until it’s not meet.

Another typical problem of sliding window is Sliding Window Maximum

Some related problems:

• Minimum Window Substring
• Longest Substring Without Repeating Characters
• Substring with Concatenation of All Words
• Longest Substring with At Most Two Distinct Characters
• Find All Anagrams in a String

Analysis