# \[LeetCode] 3. Longest Substring Without Repeating Characters \[Medium]

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

**Examples:**

Given `"abcabcbb"`, the answer is `"abc"`, which the length is 3.

Given `"bbbbb"`, the answer is `"b"`, with the length of 1.

Given `"pwwkew"`, the answer is `"wke"`, with the length of 3. Note that the answer must be a **substring**, `"pwke"` is a subsequenceand not a substring.

**Analyse:**

一開始直覺會用雙重迴圈去計算, 但結果Time Limit Exceeded...

用兩個指標: left與right去紀錄目前不重複的String,

長度length = right - left + 1;

並用一個dictionary記錄每個char最右邊的Index,

若目前right位子的char存在於dictionary,

則left = char存在於dictionary的index + 1. 直接看code會比較好理解.

**Solution:**

```swift
class Solution {
   
    func lengthOfLongestSubstring(_ s: String) -> Int {
        var maxLength = 0;
        let chars = Array(s);
        var left = 0;
        var right = 0;
        var records:[Character: Int] = [:]
        
        while(right < chars.count) {
            let char = chars[right];
            
            if(records[char] != nil) {
                left = max(records[char]! + 1, left);
            }
           
            records[char] = right;
            maxLength = max(maxLength, right - left + 1);
        
            right += 1;
        }
        
        return maxLength;
    }
}
```