leetcode -- Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

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

 But the following is not:

    1
   / \
  2   2
   \   \
   3    3

 Note:

Bonus points if you could solve it both recursively and iteratively.

递归解法

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public boolean isSymmetric(TreeNode root) {
12         // Start typing your Java solution below
13         // DO NOT write main() function
14         if(root == null){
15             return true;
16         }
17         
18         //if(root.left != null && root.right != null){
19             return cmp(root.left, root.right);
20         //}
21         
22         //return false;
23     }
24     
25     public boolean cmp(TreeNode l, TreeNode r){
26         boolean f1 = false, f2 = false;
27         if(l == null && r == null){
28             return true;
29         }
30         
31         if((l != null && r == null) ||
32             (l == null && r != null)){
33                 return false;
34             }
35 
36         if(l.val != r.val){
37             return false;
38         }
39         //if(l.left != null && r.right != null){
40             f1 = cmp(l.left, r.right);
41         //}
42         //if(l.right != null && r.left != null){
43             f2 = cmp(l.right, r.left);
44         //}
45         //if(l != null && r != null && f1 && f2){
46         //  return false;
47         //}
48         if(f1 && f2){
49             return true;
50         }
51         return false;
52         
53     }
54 }

 迭代版本(层序遍历)

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public boolean isSymmetric(TreeNode root) {
12         // Start typing your Java solution below
13         // DO NOT write main() function
14         if(root == null){
15             return true;
16         }
17         LinkedList<TreeNode> l = new LinkedList<TreeNode>(),
18             r = new LinkedList<TreeNode>();
19             
20         l.add(root.left);
21         r.add(root.right);
22         while(!l.isEmpty() && !r.isEmpty()){
23             TreeNode t1 = l.poll(), t2 = r.poll();
24             if((t1 == null && t2 != null) || (t1 != null && t2 == null)){
25                 return false;            
26             }
27             if(t1 != null){
28                 if(t1.val != t2.val){
29                     return false;
30                 }
31                 l.add(t1.left);
32                 l.add(t1.right);
33                 r.add(t2.right);
34                 r.add(t2.left);
35             }
36         }
37         return true;
38     }
39 }