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/**

* #101 Symmetric Tree

* see link : https://leetcode-cn.com/problems/symmetric-tree/

* 和 #100. Same Tree 类似 使用递归解法 O(n)

*/

public class S101 {

public static boolean isSymmetric(TreeNode root) {

if(root == null) {

return true;

}

return symmetric(root.left, root.right);

}

public static boolean symmetric(TreeNode p, TreeNode q) {

if(p == null || q == null)

return p == q ? true : false;

//

return p.val == q.val && symmetric(p.right, q.left) && symmetric(p.left, q.right);

}

public static class TreeNode {

int val;

TreeNode left;

TreeNode right;

TreeNode(int x) { val = x; }

}

public static void main(String[] args) {

//check demo

TreeNode n1 = new TreeNode(1);

TreeNode n2_left = new TreeNode(2);

TreeNode n2_right = new TreeNode(2);

n1.left = n2_left;

n1.right = n2_right;

TreeNode n3_left = new TreeNode(3);

TreeNode n3_right = new TreeNode(4);

n2_left.left = n3_left;

n2_left.right = n3_right;

n2_right.left = n3_right;

n2_right.right = n3_left;

System.out.println("isSymmetric(n1) > " + isSymmetric(n1));

}

}