Longest Substring Without Repeating Characters

问题

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.

解题思想

存在子串 $x_i, x_{i + 1}, ... , x_{j-1}$互异，现在第$x_j$个字符
若$x_j$ 与$x_i, x_{i + 1}, ... , x_{j-1}$互异，则$j = j + 1$, 否则存在$k$使得$x_k == x_j$，这时我们只需要算出此区间最大的长度$j-i+1$, 并将$i$移动到$i = k + 1$, 此时区间长度为$j - k$.

代码

// 找到c在s[from, end]中的位置
int find(char *s, int from, int end, char c) {
    int i;
    for (i = from; i <= end; ++i) {
        if (s[i] == c) {
            return i;
        }
    }
    return -1;
}

int lengthOfLongestSubstring(char* s) {
    int length_s = strlen(s);
    if (length_s < 2) {
        return length_s;
    }
    int longest = 1;
    int tmp_count = 1;
    int i, j;
    int k;

    for (i = 0, j = 1; j < length_s; ++j) {
        k = find(s, i, j - 1, s[j]);
        if (k == -1) {
            tmp_count++;
            continue;
        } else {
            if (tmp_count > longest) {
                longest = tmp_count;
            }
            i = k + 1;
            tmp_count = j - i + 1;
        }
    }
    if (tmp_count > longest) {
        longest = tmp_count;
    }
    return longest;
}Characters

性能