Binary Tree Level Order Traversal leetcode java

本文转载自查看原文 2014-08-05 03:01 3205 leetcode每题整理

题目：

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

return its level order traversal as:

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


题解：
这道题就是用传统的BFS来做。代码如下：

1     public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
2         ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
3          if(root == null)
4              return res;
5
6         LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
7         queue.add(root);
8          int curLevCnt = 1;
9          int nextLevCnt = 0;
10         ArrayList<Integer> levelres = new ArrayList<Integer>();
11
12          while(!queue.isEmpty()){
13             TreeNode cur = queue.poll();
14             curLevCnt--;
15             levelres.add(cur.val);
16
17              if(cur.left != null){
18                 queue.add(cur.left);
19                 nextLevCnt++;
20             }
21              if(cur.right != null){
22                 queue.add(cur.right);
23                 nextLevCnt++;
24             }
25
26              if(curLevCnt == 0){
27                 curLevCnt = nextLevCnt;
28                 nextLevCnt = 0;
29                 res.add(levelres);
30                 levelres = new ArrayList<Integer>();
31             }
32         }
33          return res;
34     }

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