題目：

 Given two binary trees, write a function to check if they are equal or not.

Two binary trees are considered equal if they are structurally identical and the nodes have the same value. 

解決演算法：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 

public class Solution {
    public boolean isSameTree(TreeNode p, TreeNode q) {
        if((p==null&&q!=null)||(p!=null&&q==null)){
            return false;
        }
        if(p!=null&&q!=null){
            if(!(p.val==q.val)){
                return false;
            }
        }
        if
 
(p==null&&q==null){

        }else{
            if(!isSameTree(p.left,q.left)){
                return false;
            }
            if(!isSameTree(p.right,q.right)){
                return false;
            }
        }

        return true;
    }

}