101. Symmetric Tree

最新推荐文章于 2024-01-17 14:31:46 发布

zshouyi

最新推荐文章于 2024-01-17 14:31:46 发布

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本文介绍了一种检查二叉树是否为自身镜像的方法，即是否围绕中心对称。通过递归和非递归两种方式实现，递归方法通过比较左子树和右子树来判断对称性，非递归方法则使用栈来存储节点进行逐层比较。

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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:

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

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

对比left.left, right.right && left.right, right.left，如果val相等，继续递归。注意以下几点：

1、如果root==null，返回true

2、如果left或者right等于null，不能访问其左右节点和val，否则会outstack，所以要先判断left或者right等不等于null

代码如下：

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

Non-recursive(use Stack)--460ms:

public boolean isSymmetric(TreeNode root) {
    if(root==null)  return true;
    
    Stack<TreeNode> stack = new Stack<TreeNode>();
    TreeNode left, right;
    if(root.left!=null){
        if(root.right==null) return false;
        stack.push(root.left);
        stack.push(root.right);
    }
    else if(root.right!=null){
        return false;
    }
        
    while(!stack.empty()){
        if(stack.size()%2!=0)   return false;
        right = stack.pop();
        left = stack.pop();
        if(right.val!=left.val) return false;
        
        if(left.left!=null){
            if(right.right==null)   return false;
            stack.push(left.left);
            stack.push(right.right);
        }
        else if(right.right!=null){
            return false;
        }
            
        if(left.right!=null){
            if(right.left==null)   return false;
            stack.push(left.right);
            stack.push(right.left);
        }
        else if(right.left!=null){
            return false;
        }
    }
    
    return true;
}