从上到下按层打印二叉树

问题

解决方案

• BFS 广度遍历

javascript
/**
 * 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) {
    //定义结果集数组
    let result = [];

    if (root) {
        //因为期望结果是分层数组输出的，区分层次定义父层与子层
        let parent = [];
        let children = [];
        //暂存当前层的val值
        let tmpCache = [];

        parent.push(root)
        while (!!parent.length) {
            //出栈
            let current = parent.shift();
            tmpCache.push(current.val);
             //如果当前节点存在，把左右节点暂存到children数组中
            if (current.left) {
                children.push(current.left);
            }
            if (current.right) {
                children.push(current.right);
            }
            //如果parent数组长度为0,说明当前层遍历结束，做相应的处理
            if (parent.length === 0) {
                if (tmpCache.length) {
                    result.push(tmpCache);
                }
                tmpCache = [];
                parent = children;
                children = []
            }
        }
    }
    return result;

};