/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param a, b, the root of binary trees.
     * @return true if they are identical, or false.
     */
    public boolean isIdentical(TreeNode a, TreeNode b) {
        if(a==null && b==null) return true;
        else if(a==null || b==null) return false;
        else if(a.val!=b.val) return false;
        return isIdentical(a.left,b.left) && isIdentical(a.right,b.right);
    }
}