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Leetcode: Interleaving String

Given s1s2s3, find whether s3 is formed by the interleaving of s1 and s2.

For example,
Given:
s1 = "aabcc",
s2 = "dbbca",

When s3 = "aadbbcbcac", return true.
When s3 = "aadbbbaccc", return false.

分析:动态规划, f[i][j]表示s3[0:i+j-1]是s1[0:i-1]和s2[0:j-1]的interleaving string。递推公式如下:

f[i][j] = (s1[i-1] == s3[i+j-1]) && f[i-1][j] || (s2[j-1] == s3[i+j-1]) && f[i][j-1].

代码如下:

class Solution {public:    bool isInterleave(string s1, string s2, string s3) {       if (s3.length() != s1.length() + s2.length())        return false;        vector<vector<bool>> f(s1.length() + 1,vector<bool>(s2.length() + 1, true));        for (size_t i = 1; i <= s1.length(); ++i)            f[i][0] = f[i - 1][0] && s1[i - 1] == s3[i - 1];        for (size_t i = 1; i <= s2.length(); ++i)            f[0][i] = f[0][i - 1] && s2[i - 1] == s3[i - 1];        for (size_t i = 1; i <= s1.length(); ++i)            for (size_t j = 1; j <= s2.length(); ++j)                f[i][j] = (s1[i - 1] == s3[i + j - 1] && f[i - 1][j]) || (s2[j - 1] == s3[i + j - 1] && f[i][j - 1]);        return f[s1.length()][s2.length()];    }};

本题目的递推版本很容易理解,但超时。

Leetcode: Interleaving String