20201030.md

Algorithm

101. Symmetric Tree

Description

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

Follow up: Solve it both recursively and iteratively.

Solution

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

Discuss

注意对NULL的判断

核对代码是：isSamed(left.left,right.right)&&isSamed(left.right,right.left);

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