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.
題解如下:
遞歸版
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean isSymmetric(TreeNode root) {
return isMirror(root,root);
}
public boolean isMirror(TreeNode t1,TreeNode t2) {
if(t1==null && t2==null)
return true;
if(t1 == null || t2 == null)
return false;
return (t1.val == t2.val)
&& isMirror(t1.left,t2.right)
&& isMirror(t1.right,t2.left);
}
}
非遞歸版
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean isSymmetric(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<>();
queue.add(root);
queue.add(root);
while(!queue.isEmpty()) {
TreeNode t1 = queue.poll();
TreeNode t2 = queue.poll();
if(t1 == null && t2 == null)
continue;
if(t1 == null || t2 == null)
return false;
if(t1.val != t2.val)
return false;
queue.add(t1.left);
queue.add(t2.right);
queue.add(t1.right);
queue.add(t2.left);
}
return true;
}
}