LeetCode #3 Longest Substring Without Repeating Characters

LeetCode #3 Longest Substring Without Repeating Characters

Question

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 subsequence and not a substring.

Solution

Approach #1

class Solution {
    func lengthOfLongestSubstring(_ s: String) -> Int {
        let arr = Array(s.utf16)
        let count = arr.count
        var map: [UTF16.CodeUnit : Int] = [:]
        var i = 0
        var j = 0
        var maxLength = 0
        while i < count, j < count {
            if let index = map[arr[j]] {
                i = max(i, index + 1)
            }
            map[arr[j]] = j
            j += 1
            maxLength = max(maxLength, j - i)
        }
        return maxLength
    }
}

Time complexity: O(n).

Space complexity: O(m). m is the size of map.

注意，以下代码的思路和上面完全一样，但结果是“Time Limit Exceeded”

class Solution {
    func lengthOfLongestSubstring(_ s: String) -> Int {
        var map: [Character : Int] = [:]
        var i = 0
        var j = 0
        var maxLength = 0
        let count = s.characters.count
        while i < count, j < count {
            let endChar = s[s.index(s.startIndex, offsetBy: j)]
            if let index = map[endChar] {
                i = max(i, index + 1)
            }
            map[endChar] = j
            j += 1
            maxLength = max(maxLength, j - i)
        }
        return maxLength
    }
}

原因是，String 以 String.Index 取下标比较耗时，不如数组的随机访问效率高。参见：http://www.cnblogs.com/silence-cnblogs/p/6877463.html

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