LeetCode 102. Binary Tree Level Order Traversal

原题链接在这里：https://leetcode.com/problems/binary-tree-level-order-traversal/

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,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7

return its level order traversal as:

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

题解：

是一道BFS，利用queue，先进先出，并利用curCount来判断是否走完了当前一层。当curCount = 0 时表示当前一层走完，要把当前list加入res里，再刷新list，问题是不要忘记同时刷新curCount和nextCount.

Time Complexity: O(n). Space: O(n).

AC Java:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if(root == null){
            return res;
        }
        
        LinkedList<TreeNode> que = new LinkedList<TreeNode>();
        que.add(root);
        int curCount = 1;
        int nextCount = 0;
        List<Integer> item = new ArrayList<Integer>();
        while(!que.isEmpty()){
            TreeNode tn = que.poll();
            item.add(tn.val);
            curCount--;
            
            if(tn.left != null){
                que.add(tn.left);
                nextCount++;
            }
            if(tn.right != null){
                que.add(tn.right);
                nextCount++;
            }
            
            if(curCount == 0){
                res.add(new ArrayList<Integer>(item));
                item = new ArrayList<Integer>();
                curCount = nextCount;
                nextCount = 0;
            }
        }
        return res;
    }
}

 类似的有Binary Tree Level Order Traversal II, Binary Tree Zigzag Level Order Traversal, Binary Tree Vertical Order Traversal, Binary Tree Right Side View, Find Largest Value in Each Tree Row, Check Completeness of a Binary Tree.