LeetCode(102):Binary Tree Level Order Traversal

Binary Tree Level Order Traversal: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},

    3
   / 
  9  20
    /  
   15   7

return its level order traversal as:

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

题意：二叉树的层次遍历。

思路：借助队列进行遍历。

代码：

public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        if(root==null) return new ArrayList<List<Integer>>();
             List<List<Integer>> result = new ArrayList<List<Integer>>();
             Queue<TreeNode> queue = new LinkedList<TreeNode>();
             int levCount  = 1;
             int preLevCount = 1;
             queue.add(root);
             while(!queue.isEmpty()){
                 levCount = 0;
                 List<Integer> temp = new ArrayList<Integer>();
                 for(int i=0;i<preLevCount;i++){
                     TreeNode tree = queue.poll();
                     temp.add(tree.val);
                     if(tree.left!=null){
                         queue.add(tree.left);
                         levCount++;
                     }
                     if(tree.right!=null){
                         queue.add(tree.right);
                         levCount++;
                     }
                 }
                 preLevCount = levCount;
                 result.add(temp);
             }
             return result;
    }
}