Symmetric Tree（对称二叉树）

来源：https://leetcode.com/problems/symmetric-tree

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

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / 
  2   2
 /  / 
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / 
  2   2
      
   3    3

递归：

1. 若根节点为null，返回true

2. 若根节点不为null，对其左右子树进行递归地比较：若两个节点都为null，返回true；若一个为null，另一个不为null，返回false；若两个节点（A,B）值相等，则返回 (A.left, B.right) 的比较结果 && (A.right, B.left) 的比较结果。

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean compareLR(TreeNode left, TreeNode right) {
        if(left == null && right == null) {
            return true;
        }
        if((left == null && right != null) || (left != null && right == null)) {
            return false;
        }
        if(left.val == right.val) {
            return compareLR(left.right, right.left) && compareLR(left.left, right.right);
        }
        return false;
    }
    public boolean isSymmetric(TreeNode root) {
        if(root == null) {
            return true;
        }
        return compareLR(root.left, root.right);
    }
}

Python

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def isSymmetrical(self, pRoot):
        def comRoot(left, right):
            if not left:
                return not right
            if not right or (left.val != right.val):
                return False
            return comRoot(left.right, right.left) and comRoot(left.left, right.right)
        if not pRoot:
            return True
        return comRoot(pRoot.left, pRoot.right)

迭代：

1. 若根节点为null，返回true

2. 若根节点不为null，将其左右两个子节点入队列

3. 从队列中取出两个节点，若两个节点都为null，返回true；若一个为null，另一个不为null，返回false；若两个节点（A,B）值相等，将他们的两个子节点按照A.left，B.right，A.right，B.left的顺序入队列，重复执行该步，直到队列中不再有节点。