## Description

https://leetcode.com/problems/minimum-window-substring/

Given two strings `s`

and `t`

of lengths `m`

and `n`

respectively, return *the minimum window substring of *

`s`

*such that every character in*

`t`

*(*

**including duplicates**) is included in the window. If there is no such substring*, return the empty string*

`""`

*.*

The testcases will be generated such that the answer is **unique**.

A **substring** is a contiguous sequence of characters within the string.

**Example 1:**

Input:s = "ADOBECODEBANC", t = "ABC"Output:"BANC"Explanation:The minimum window substring "BANC" includes 'A', 'B', and 'C' from string t.

**Example 2:**

Input:s = "a", t = "a"Output:"a"Explanation:The entire string s is the minimum window.

**Example 3:**

Input:s = "a", t = "aa"Output:""Explanation:Both 'a's from t must be included in the window. Since the largest window of s only has one 'a', return empty string.

**Constraints:**

`m == s.length`

`n == t.length`

`1 <= m, n <= 10`

^{5}`s`

and`t`

consist of uppercase and lowercase English letters.

**Follow up:** Could you find an algorithm that runs in `O(m + n)`

time?

## Explanation

Use two pointers sliding window. Increase the right pointer to meet the target string, and adjust the left pointer to see where is the place to have a minimum substring to have the target string.

## Python Solution

```
class Solution:
def minWindow(self, s: str, t: str) -> str:
counter_t = {}
counter_s = {}
for c in t:
counter_t[c] = counter_t.get(c, 0) + 1
i = 0
j = 0
left = -1
right = -1
valid = 0
for i in range(len(s)):
while j < len(s) and valid < len(counter_t):
counter_s[s[j]] = counter_s.get(s[j], 0) + 1
if s[j] in counter_t and counter_s[s[j]] == counter_t[s[j]]:
valid += 1
j += 1
if valid == len(counter_t):
if left == -1 or j - i < right - left:
left = i
right = j
counter_s[s[i]] -= 1
if s[i] in counter_t and counter_s[s[i]] == counter_t[s[i]] - 1:
valid -= 1
if left == -1:
return ""
return s[left : right]
```

- Time Complexity: O(N).
- Space Complexity: O(N).

where Q is the length of queries and N is the length of colors.

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