Symmetric Tree

最新推荐文章于 2024-01-17 14:31:46 发布

转载最新推荐文章于 2024-01-17 14:31:46 发布 · 49 阅读

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原文链接：http://www.cnblogs.com/RazerLu/p/3532674.html

本文介绍了如何使用递归和非递归方法来判断二叉树是否为对称树，包括代码实现和核心逻辑解析。

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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 is symmetric:

But the following is not:

参考 http://www.cnblogs.com/feiling/p/3251061.html & http://fisherlei.blogspot.com/2013/01/leetcode-symmetric-tree.html

非递归解法：按层遍历，每一层检查一下是否对称。

/**
 * Definition for binary tree
 * 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;
        }
        
        LinkedList<TreeNode> l = new LinkedList<TreeNode>(),
        LinkedList<TreeNode> r = new LinkedList<TreeNode>();
         
         l.add(root.left);
         r.add(root.right);
         
         while(!l.isEmpty() && !r.isEmpty()) {
             TreeNode t1 = l.poll(); TreeNode t2 = r.poll();
             
             if((t1 == null && t2 != null)||(t2 == null && t1 != null)){
                 return false;
             }
             
             if( t1 != null){
                 if(t1.val != t2.val){
                     return false;
                 }
                 
                 l.add(t1.left);
                 l.add(t1.right);
                 r.add(t2.right);
                 r.add(t2.left);
             }
             
         }
         
         return true;
    }
}

递归解法：

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public boolean isSymmetric(TreeNode root) {
12         if(root == null)
13             return true;
14         
15         return checkSymmetric(root.left, root.right);
16     }
17     
18     private boolean checkSymmetric(TreeNode left, TreeNode right){
19         if(left == null && right == null)
20             return true;
21         
22         if((left == null && right != null) || (left != null && right == null))
23             return false;
24         
25         if(left.val != right.val)
26             return false;
27         
28         return checkSymmetric(left.left, right.right) && checkSymmetric(left.right, right.left);
29     }
30 }

其中左子树和右子树对称的条件：

两个节点值相等，或者都为空
左节点的左子树和右节点的右子树对称

左节点的右子树和右节点的左子树对称

/**
 * Definition for binary tree
 * 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;
        else
            return check(root.left, root.right);
    }
    
    private boolean check(TreeNode left, TreeNode right){
        if(left == null) return right == null;
        if(right == null) return left == null;
        
        if(left.val != right.val)
            return false;
        else
            return check(left.left, right.right) && check(left.right, right.left);
    }
}