檢查兩棵二叉樹是否等價。等價的意思是說，首先兩棵二叉樹必須擁有相同的結構，并且每個對應位置上的節點上的數都相等。

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/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
 
class Solution {
public:
    /**
     * @aaram a, b, the root of binary trees.
     * @return true if they are identical, or false.
     */
 bool isIdentical(TreeNode* a, TreeNode* b) {
        // Write your code here
        if(a==NULL&&b==NULL)
            return true;
        if((a!=NULL&&b==NULL)||(a==NULL&&b!=NULL))
            return false;
        if(a->val==b->val)
        {
            return isIdentical(a->left, b->left) && isIdentical(a->right, b->right);
        }
        else{
            return false;
        }
     }
};