[One question per day] Implement strStr ()

leet-code，实现 strStr()：https://leetcode-cn.com/problems/implement-strstr/

Solution 1 Double pointer

Double pointer O (m + n)
traverses every character of haystack as the beginning
0 ms 6.9 MB

class Solution {
public:
    int strStr(string haystack, string needle) {
        if (needle.size() == 0) {
            return 0;
        }
        if (haystack.size() < needle.size()) {
            return -1;
        }

        size_t i = 0, j = 0;
        int ret = -1;
        while (i < haystack.size() && j < needle.size()) {
            if (haystack[i] == needle[j]) {
                if (ret == -1) {
                    ret = i;
                }
                ++i; ++j;
            } else {
                if (ret != -1) {
                    j = 0;
                	// 回溯
                    i = ret + 1;
                    ret = -1;
                } else {
                    ++i;
                }
            }
        }
        // i-ret 如果不和 needle.size() 相等，则不存在
        if ((i - ret) != needle.size()) {
            ret = -1;
        }

        return ret;
    }
};

Solution 2 Use map

Use map
to split haystack into a string of length needle.size (), save it as a key in the map, and index as the second parameter
12 ms 8 MB

class Solution {
public:
    int strStr(string haystack, string needle) {
        if (needle.size() == 0) {
            return 0;
        }
        if (haystack.size() < needle.size()) {
            return -1;
        }

        map<string, size_t> m;
        for (size_t i = 0; i <= haystack.size() - needle.size(); ++i) {
            string key = haystack.substr(i, needle.size());
            // 如果没有这个 key 才添加到 map 中（也就是说只返回第一次出现的位置）
            if (m.find(key) == m.end()) {
                m[key] = i;
            }
        }

        if (m.find(needle) != m.end()) {
            return m[needle];
        }
        return -1;
    }
};

Solution 3 KMP

TODO: Not yet learned.

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