LeetCode 003 Longest Substring Without Repeating Characters O(n)做法

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.
用HashMap加快查询速度。HashMap.containsKey复杂度O(1)，HashMap.containsValue复杂度O(n)。class Solution {
public int lengthOfLongestSubstring(String s) {
HashMap<Character, Integer> map = new HashMap<Character, Integer>();
int max = 0;
for(int i = 0, j = 0; i < s.length(); i++) {
if(map.containsKey(s.charAt(i))) {
// 这里取max是因为map中的元素加入之后一直存在
// map.get能获得最后一次出现的位置
j = Math.max(j, map.get(s.charAt(i)) + 1);
}
map.put(s.charAt(i), i);
max = Math.max(max, i - j + 1);
}
return max;
}
}