leetcode101~Symmetric Tree

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

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

private boolean check(TreeNode root1, TreeNode root2) {
if(root1 == null && root2 == null) {
return true;
}
if(root1 == null || root2 == null) {
return false;
}
if(root1.val != root2.val) {
return false;
}
return check(root1.left,root2.right) && check(root1.right,root2.left);
}

//非递归解法
public boolean isSymmetric2(TreeNode root) {
if(root == null) return true;
if(root.left==null && root.right==null) return true;
if(root.left ==null || root.right == null) return false;

LinkedList<TreeNode> q1 = new LinkedList<>();
LinkedList<TreeNode> q2 = new LinkedList<>();
q1.add(root.left);
q2.add(root.right);

while(!q1.isEmpty() && !q2.isEmpty()) {
TreeNode n1 = q1.poll();
TreeNode n2 = q2.poll();

if(n1.val!=n2.val) return false;
if((n1.left==null &&n2.right!=null) || (n1.left!=null&&n2.right==null)){
return false;
}
if((n1.right==null &&n2.left!=null) || (n1.right!=null&&n2.left==null)){
return false;
}

if(n1.left!=null && n2.right!=null) {
q1.add(n1.left);
q2.add(n2.right);
}
if(n1.right!=null && n2.left!=null) {
q1.add(n1.right);
q2.add(n2.left);
}
}
return true;
}
}