Binary Tree ZigZag Level Order Traversal leetcode java

タイトル:
 
Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).
For example: Given binary tree {3,9,20,#,#,15,7} ,

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]



  ：
     BFS，   flag      reverse，        reverse       。
    ：

 1     
public ArrayList> zigzagLevelOrder(TreeNode root) {
 2         ArrayList> res = 
new ArrayList>();
 3         
 4         
if(root==
null)
 5             
return res;
 6         
 7         LinkedList queue = 
new LinkedList();
 8         queue.add(root);
 9         
10         
int num = 0;
11         
boolean reverse = 
false;
//
a flag
12 
        
13         
while(!queue.isEmpty()){
14             num = queue.size();
15             ArrayList levelres = 
new ArrayList();
16             
17             
for(
int i = 0; i18                 TreeNode node = queue.poll();
19                 levelres.add(node.val);
20                 
21                 
if(node.left!=
null)
22                     queue.add(node.left);
23                 
if(node.right!=
null)
24                     queue.add(node.right);
25             }
26             
27             
if(reverse){
28                 Collections.reverse(levelres);
29                 reverse = 
false;
30             }
else{
31                 reverse = 
true;
32             }
33             res.add(levelres);
34         }
35         
36         
return res;
37     }

 1     
public ArrayList> zigzagLevelOrder(TreeNode root) {
 2         ArrayList> res = 
new ArrayList>();
 3         
if(root == 
null)
 4             
return res;
 5         
 6         LinkedList queue = 
new LinkedList();
 7         queue.add(root);
 8         Boolean reverse = 
false;
 9         
int nextlevel = 0;
10         
int currlevel = 1;
11         ArrayList tmp = 
new ArrayList();
12         
while(!queue.isEmpty()){
13                 TreeNode t = queue.poll();
14                 tmp.add(t.val);
15                 currlevel--;
16                 
17                 
if(t.left!=
null){
18                     queue.add(t.left);
19                     nextlevel++;
20                 }
21                 
22                 
if(t.right!=
null){
23                     queue.add(t.right);
24                     nextlevel++;
25                 }
26             
27             
if(currlevel == 0){
28                 currlevel = nextlevel;
29                 nextlevel = 0;
30                 
if(reverse){
31                     Collections.reverse(tmp);
32                     reverse = 
false;
33                 }
else{
34                     reverse = 
true;
35                 }
36                 res.add(tmp);
37                 tmp = 
new ArrayList();
38             }
39         }
40         
41         
return res;
42     }