Identical Binary Tree

题目：

Check if two binary trees are identical. Identical means the two binary trees have the same structure and every identical position has the same value.

分析：

这题目从程序的逻辑上讲不复杂，但是比较考验细致的事情，第二次做的时候我忘记了判断(a == NULL || b == NULL)的情况。

解法：

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) {
            return false;
        }
        if (a->val != b->val) {
            return false;
        }
        return (isIdentical(a->left, b->left) && isIdentical(a->right, b->right) );
    }
};