101. Symmetric Tree

题目分析

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

Note:
Bonus points if you could solve it both recursively and iteratively.

代码

递归实现

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