题目描述:
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
Constraints:
0 <= s.length <= 5 * 104
s consists of English letters, digits, symbols and spaces.
Time complexity: O(n)
同向双指针:
1、指针j 遍历字符,维护一个[i,j]区间始终维持以j结尾的无重复字符的最长子串,用哈希表记录[i,j]区间的每个元素的个数
2、每当j往前走一格时,直接将j位置的元素加入到哈希表中,然后判断j位置的元素在[i,j]区间中是否有重复,若有重复则i往前走一步,并将i位置的元素数量减1,当[i,j]中确保不存在重复元素时,则更新res:比较当前长度 j-i+1 和原长度 res 取最大值。
class Solution {
public int lengthOfLongestSubstring(String s) {
int res = 0;
int[] a = new int[256];
int len = s.length();
for(int i = 0,j = 0;j<len;j++){
int index = s.charAt(j);
a[index]++;
while(a[index] > 1){
a[s.charAt(i++)]--;
}
res = Math.max(res, j-i+1);
}
return res;
}
}