【LeetCode】3. Longest Substring Without Repeating Characters

Difficulty: 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 subsequence and not a substring.

分析
假设子串里含有重复字符，则父串一定含有重复字符，单个子问题就可以解决父问题，因此可以使用贪心法。
跟动规不同，动规里，单个子问题只能影响父问题，不足以解决父问题。
从左往右扫描，当遇到重复字母时，以上一个重复字母的index+1，作为新的搜索起点位置，直到最后一个字母，
时间复杂度为O（n）

class Solution {
public:
int lengthOfLongestSubstring(string s) {
const int ASCII_MAX = 255;
int last[ASCII_MAX];
int start = 0;
fill(last, last + ASCII_MAX, -1);
int max_len = 0;
for (int i = 0; i < s.size(); i++) {
if (last[s[i]] >= start) {
max_len = max(i - start, max_len);
start = last[s[i]] + 1;
}
last[s[i]] = i;
}
return max((int)s.size() - start, max_len);
}
};