Leetcode Symmetric Tree

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

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following is not:

    1
   / \
  2   2
   \   \
   3    3 

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

confused what "{1,#,2,3}" means? 

 

OJ's Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

   1
  / \
 2   3
    /
   4
    \
     5

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".

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解题思路：

1. 递归

2. iterative,建立两个queue, 一个从左往右，一个从右往左，镜像比较

仔细考虑每种情况，很容易漏掉。

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Java code :

recursion:

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root == null) {
           return true;
        }
        return isSymmetric(root.left, root.right);
    }
    
    public boolean isSymmetric(TreeNode l, TreeNode r) {
        if(l == null && r == null){
            return true;
        }else if(l == null || r == null) {
            return false;
        }
        if(l.val != r.val ){
            return false;
        }
        if(!isSymmetric(l.left,r.right)) {
            return false;
        }
        if(!isSymmetric(l.right,r.left)) {
            return false;
        }
        return true;
    }
}

iterative

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        //use iterative
        if(root == null){
            return true;
        }
        Queue<TreeNode> left = new LinkedList<TreeNode>();
        Queue<TreeNode> right = new LinkedList<TreeNode>();
        left.add(root.left);
        right.add(root.right);
        while(!left.isEmpty() && !right.isEmpty()) {
            TreeNode l = left.remove();
            TreeNode r = right.remove();
            if(l== null && r== null) {
                continue;
            }else if(l == null || r == null){
                return false;
            }
            
            if(l.val != r.val) {
                return false;
            }else {
                left.add(l.left);
                left.add(l.right);
                right.add(r.right);
                right.add(r.left);
            }
        }
        return true;
    }
}

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Reference:

1. http://www.programcreek.com/2014/03/leetcode-symmetric-tree-java/