leetcode101.对称二叉树
给定一个二叉树，检查它是否是镜像对称。
递归做法：
/**
* 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) {
if(root == null) return true;
return isMirror(root.left , root.right);
}
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);
}
}
非递归做法：
/**
* 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) {
if(root == null) return true;
Queue<TreeNode> q = new LinkedList<>();
q.add(root.left);
q.add(root.right);
while(!q.isEmpty()){
TreeNode t1 = q.poll();
TreeNode t2 = q.poll();
if(t1 == null && t2 == null){
continue;
}
if(t1 == null || t2 == null){
return false;
}
if(t1.val != t2.val){
return false;
}
q.add(t1.left);
q.add(t2.right);
q.add(t1.right);
q.add(t2.left);
}
return true;
}
} 
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