class Solution {
    public boolean isInterleave(String s1, String s2, String s3) {
        int m = s1.length(), n = s2.length();
        if (m + n != s3.length()) {
            return false;
        }
        boolean[][] f = new boolean[m + 1][n + 1];
        f[0][0] = true;
        for (int i = 0; i <= m; ++i) {
            for (int j = 0; j <= n; ++j) {
                int k = i + j - 1;
                if (i > 0 && s1.charAt(i - 1) == s3.charAt(k)) {
                    f[i][j] = f[i - 1][j];
                }
                if (j > 0 && s2.charAt(j - 1) == s3.charAt(k)) {
                    f[i][j] |= f[i][j - 1];
                }
            }
        }
        return f[m][n];
    }
}