从上到下打印二叉树 II

从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。

例如: 给定二叉树: [3,9,20,null,null,15,7]

1
2
3
4
5

返回其层次遍历结果：

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

1
2
3
4
5

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrder = function(root) {
    if(root === null) return []
    let queue = []
    let ans = []
    queue.push([root])

    while (queue.length) {
        let curr = queue.shift()
        if (curr.length) {
            let curAns = []
            let prev = []
            for (let i = 0; i < curr.length; i++) {
                curAns.push(curr[i].val)

                if (curr[i].left) {
                    prev.push(curr[i].left)
                }
                if (curr[i].right) {
                    prev.push(curr[i].right)
                }
            }
            ans.push(curAns)
            queue.push(prev)
        }
    }
    return ans
};

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38

来源：力扣（LeetCode）链接：https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof

← 二叉树的镜像从上到下打印二叉树 III→