24. Interleaving String

Link to the problem

Description

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

Example

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

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

Idea

Use dynamic programming, for s1[:i] and s2[:j], we can check if they can be interleaved to s3[:i + j] by asking s1[:i - 1], s2[:j], and s1[:i], s2[:j - 1]. Space can be optimized by storing two rolling rows.

Solution

class Solution {
public:
    bool isInterleave(string s1, string s2, string s3) {
        int m = s1.size(), n = s2.size();
        if (s3.size() != m + n) return false;
        vector dp(n + 1, false);
        dp[0] = true;
        for (int j = 1; j <= n; ++j) {
            dp[j] = dp[j - 1] && s2[j - 1] == s3[j - 1];
        }
        for (int i = 1; i <= m; ++i) {
            vector dp2(n + 1, false);
            for (int j = 0; j <= n; ++j) {
                if (i && dp[j] && s1[i - 1] == s3[i + j - 1]) dp2[j] = true;
                if (j && dp2[j - 1] && s2[j - 1] == s3[i + j - 1]) dp2[j] = true;
            }
            dp = dp2;
        }
        return dp[n];
    }
};

101 / 101 test cases passed.
Runtime: 4 ms