https://leetcode.com/problems/interleaving-string/

class Solution {
public:
    bool isInterleave(string s1, string s2, string s3) {
        int l1 = s1.length();
        int l2 = s2.length();
        int l3 = s3.length();
        vector<vector<int>> f(l1 + 1, vector<int>(l2 + 1, 0));
        
        if (l3 != l1 + l2) {
            return false;
        }
        
        f[0][0] = l1 == 0 && l2 == 0 && l3 != 0 ? false : true;
        
        for (int i = 1; i <= l1; i++) {
            f[i][0] = f[i-1][0] && s1[i-1] == s3[i-1];    
        }
        
        for (int j = 1; j <= l2; j++) {
            f[0][j] = f[0][j-1] && s2[j-1] == s3[j-1];    
        }
        
        for (int i = 1; i <= l1; i++) {
            for (int j = 1; j <= l2; j++) {
                if (f[i-1][j] && s1[i-1] == s3[i+j-1]) {
                    f[i][j] = f[i-1][j];
                } 
                if (f[i][j-1] && s2[j-1] == s3[i+j-1]) {
                    f[i][j] = f[i][j-1];
                }
            }
        }
        
        return f[l1][l2];
    }
};