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.

思路：遞迴，判斷根結點是否相同，不同返回false，相同的話再遞迴判斷左子樹和右子樹

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSameTree(TreeNode* p, TreeNode* q) {
        if(p==0){
            return q==0;
        }
        if(q==0){
            return false;
        }
        return (p->val==q->val) && isSameTree(p->left,q->left) && isSameTree(p->right,q->right);
    }
};