LeetCode 之 Symmetric Tree

原题：

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

这个题利用递归非常好解，每次判定两个节点，看这两个节点是否对称就行。假如当前判定的是node1和node2：

1 如果node1和node2都为null，对称

2 如果node1和node2都不为null，如果node1->val == node2->val，还不一定对称，需要判定node1的左和node2的右是否对称；node1的右和node2的左是否对称。

3 如果node1和node2有一个不为null，则不对称

代码（32ms）：

class Solution {
public:
    bool isSymmetric(TreeNode *root) {
        if(!root) return true;
        return two_compare(root->left , root->right);
    }
    
    bool two_compare(TreeNode * leftTree , TreeNode * rightTree){
        
        if(leftTree && rightTree){
            if(leftTree->val != rightTree->val){
                return false;
            }
            return ( two_compare(leftTree->left,rightTree->right) && two_compare(leftTree->right,rightTree->left) );
        }
        else if(!leftTree && !rightTree){
            return true;
        }
        else {
            return false;
        }
        
    }
    
};