Binary Tree Level Order Traversal

题目

Given a binary tree, return thelevel ordertraversal 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]
]

分析

1. using BFS(Breadth-first search) to solve this problem
2. why we need bother to assign q.size() to a new variable ? Can we just change from for(int i = 0; i <levelNodeNum; i++)to for(int i = 0; i <q.size(); i++)

图解

解题

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) return res;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while(!q.isEmpty()) {
            int levelNodeNum = q.size();
            List<Integer> currLevel = new ArrayList<>();
            for(int i = 0; i < levelNodeNum; i++) {
                TreeNode currNode = q.poll();
                currLevel.add(currNode.val);
                if (currNode.left != null)
                    q.add(currNode.left);
                if (currNode.right != null)
                    q.add(currNode.right);
            }
            res.add(currLevel);
        }
        return res;
    }
}