Leetcode(101) Symmetric Tree

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Description

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
3
4
5
    1
   / \
  2   2
 / \ / \
3  4 4  3

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

1
2
3
4
5
  1
 / \
2   2
 \   \
 3    3

解法

比较一棵二叉树是否为对称二叉树，即比较根节点的左右子树是否对称。在比较两棵树是否对称时，用递归的思想考虑，先比较根节点是否相等，然后再比较树1的左子树是否等于树2的右子树，树1的右子树是否等于树2的左子树。

具体代码如下：

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class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return helper(root.left, root.right);
    }

    boolean helper(TreeNode left, TreeNode right) {
        if (left == null && right == null) {
            return true;
        } else if (left != null && right != null) {
            if (left.val == right.val) {
                return helper(left.left, right.right) && helper(left.right, right.left);
            }
            return false;
        }
        return false;
    }
}