leetcode 3. Longest Substring Without Repeating Characters

题目

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.

思路：利用一个exist数组保存字符是否出现（假设char set是ascii），从前向后遍历数组，如果遇到已存在的字符，应该回退到这个字符上次出现的下一个位置从新开始统计，同时注意exist数组的同步更新。

代码public class Solution {
public int lengthOfLongestSubstring(String s) {
if(s.equals("")){
return 0;
}
int res = 1;
int start = 0;
boolean exist[] = new boolean[128];
for(int i = 0;i < 128; i++){
exist[i] = false;
}
for(int i = 0;i < s.length(); i++){
char c = s.charAt(i);
if(exist[c]){
for(int k = start;k < i; k++){
if(s.charAt(k)==c){
start = k+1;
break;
}
exist[s.charAt(k)] = false;
}
}else{
exist[c] = true;
}
if((i-start+1) > res){
res = i - start + 1;
}
}

return res;
}
}