[LeetCode] Symmetric Tree

Question：

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

1、题型分类：

2、思路：比较left.left与right.right和left.right与right.left,如此递归下去

3、时间复杂度：

4、代码：

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

 

5、优化：

　　非递归

6、扩展：