[leetcode]101. Symmetric Tree对称树

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

 

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

 

题目

给定二叉树，判断其是否左右对称

 

思路

DFS

 

代码

/* 
Time:O(n)，
Space: O(logn) 即 O(h) 树的deepest hight
*/
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return isSymmetric(root.left, root.right);
    }
    private static boolean isSymmetric(TreeNode p, TreeNode q) {
        if (p == null && q == null) return true;   // 终止条件
        if (p == null || q == null) return false;  // 终止条件
        return p.val == q.val      // 三方合并
                && isSymmetric(p.left, q.right)
                && isSymmetric(p.right, q.left);
    }
}