Given a string s, find the length of the longest substring without repeating characters.

### Example 1:

```
Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.
```

### Example 2:

```
Input: s = "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.
```

### Example 3:

```
Input: s = "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3.
Notice that the answer must be a substring, "pwke" is a subsequence and not a substring.
```

### Example 4:

```
Input: s = ""
Output: 0
```

### Solution

Time complexity : O(n)

```
var lengthOfLongestSubstring = function(s) {
let start = 0;
let output = 0;
let map = {}
// In each loop currentLetter gets saved in the map
// with its index+1 as the value
for(let end = 0; end < s.length; end++) {
const currentLetter = s.charAt(end);
// If we have encountered currentLetter before,
// we move the start pointer to the value
// of currentLetter in the map or
// keep the start pointer the same depending
// on which value is greater
if (map[currentLetter]!==undefined) {
// This prevents the start pointer from decreasing
start = Math.max(map[currentLetter], start);
}
output = Math.max(output, end - start + 1);
// Add 1 to the index to prevent the currentLetter
// from being included in the window when
// currentLetter is encountered again
map[currentLetter]=end+1;
}
return output;
};
```

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