[leetcode.com]算法题目 - Same Tree

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.

/**
 * Definition for binary tree
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (NULL==p && NULL==q) return true;
        
        if (NULL!=p && NULL==q) return false;
        
        if (NULL !=q && NULL==p) return false;
        
        if (p->val == q-> val)
            return (isSameTree(p->left,q->left) && isSameTree(p->right,q->right));
        else
            return false;
    }
};

思路：采用递归的方法，先判断（1）p和q都为NULL，则返回true；（2）p和q一个是NULL，一个不是NULL，则返回false；（3）如果（1）和（2）都不是，则证明两个tree都不是NULL，判断p->value是否等于q->value，若不等于，则返回false；若等于，则返回（他们的左子树相等 and 他们的右子树相等）。