Given three strings: s1, s2, s3, determine whether s3 is formed by the interleaving of s1 and s2.

Have you met this question in a real interview? Yes
Example
For s1 = "aabcc", s2 = "dbbca"

When s3 = "aadbbcbcac", return true.
When s3 = "aadbbbaccc", return false.
Challenge 
O(n2) time or better

public boolean isInterleave(String s1, String s2, String s3) {
        // write your code here
        //state
        int m = s1.length();
        int n = s2.length();
        int k = s3.length();
        if (m + n != k) {
            return false;
        }
        boolean[][] f = new boolean[m + 1][n + 1];
        f[0][0] = true;
        //initialize
        for (int i = 1; i <= m; i++) {
            if (s1.charAt(i - 1) == s3.charAt(i - 1) && f[i - 1][0]) {
                f[i][0] = true;
            }
        }
         for (int i = 1; i <= n; i++) {
            if (s2.charAt(i - 1) == s3.charAt(i - 1) && f[0][i - 1]) {
                f[0][i] = true;
            }
        }
        
        //function
         for (int i = 1; i <= m; i++) {
             for (int j = 1; j <= n; j++) {
            if (s1.charAt(i - 1) == s3.charAt(i + j - 1) && f[i - 1][j] || s2.charAt(j - 1) == s3.charAt(i + j - 1) && f[i][j - 1]) {
                f[i][j] = true;
            }
            }
        }
        
        return f[m][n];
    }