> For the complete documentation index, see [llms.txt](https://liuyang89116.gitbook.io/my-leetcode-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://liuyang89116.gitbook.io/my-leetcode-book/chapter_2_string/sliding-window-problems.md).

# Sliding Window Problems

> Reference: <https://discuss.leetcode.com/topic/30941/here-is-a-10-line-template-that-can-solve-most-substring-problems/37>

## Template

* For most substring problem, we are given a string and need to find a substring of it which satisfy some restrictions. A general way is to use a hashmap assisted with two pointers.

```c
int findSubstring(string s){
        vector<int> map(128,0);
        int counter; // check whether the substring is valid
        int begin=0, end=0; //two pointers, one point to tail and one  head
        int d; //the length of substring

        for() { /* initialize the hash map here */ }

        while(end<s.size()){

            if(map[s[end++]]-- ?){  /* modify counter here */ }

            while(/* counter condition */){ 

                 /* update d here if finding minimum*/

                //increase begin to make it invalid/valid again

                if(map[s[begin++]]++ ?){ /*modify counter here*/ }
            }  

            /* update d here if finding maximum*/
        }
        return d;
 }
```

## Algorithm

1. Use two pointers: **start** and **end** to represent a window.
2. Move **end** to find a valid window.
3. When a valid window is found, move **start** to find a smaller window.