leetcode---interleaving-string---dp

Given s1, s2, s3, 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.

class Solution {public:    bool isInterleave(string s1, string s2, string s3)     {        int n1 = s1.size();        int n2 = s2.size();        int n3 = s3.size();        if(n1 + n2 != n3)            return false;        vector<vector<bool>> dp(n1+1, vector<bool>(n2+1, false));        for(int i=0; i<=n1; i++)        {            for(int j=0; j<=n2; j++)            {                if(i==0 && j==0)                    dp[i][j] = true;                else if(i == 0)                    dp[i][j] = dp[i][j-1] && (s3[j-1] == s2[j-1]);                else if(j == 0)                    dp[i][j] = dp[i-1][j] && (s3[i-1] == s1[i-1]);                else                    dp[i][j] = (dp[i][j-1] && (s3[i+j-1] == s2[j-1])) || (dp[i-1][j] && (s3[i+j-1] == s1[i-1]));            }        }        return dp[n1][n2];    }};