2021年10月14日（102. 二叉树的层序遍历）

首发于 语雀文档@blueju

代码：

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */
function levelOrder(root: TreeNode | null): number[][] {
    if (root === null) return []
    const queue: TreeNode[] = []
    const levelOrder: number[][] = []
    queue.push(root)
    levelOrder.push([root.val])
    while (queue.length !== 0) {
        const levelQueueLength = queue.length
        let currentLevelQueue: number[] = []
        for (let i = 0; i < levelQueueLength; i++) {
            const node = queue.shift()
            if (node.left != null) {
                queue.push(node.left);
                currentLevelQueue.push(node.left.val);
            }
            if (node.right != null) {
                queue.push(node.right);
                currentLevelQueue.push(node.right.val);
            }
        }
        currentLevelQueue.length && levelOrder.push(currentLevelQueue)
    }
    return levelOrder
};

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */
function levelOrder(root: TreeNode | null): number[][] {
    if (root === null) return []
    /** 队列 */
    let queue: TreeNode[] = []
    /** 层序遍历 */
    let levelOrder: number[][] = []
    /** 添加第一层 */
    queue.push(root)
    levelOrder.push([root.val])
    /** 当队列不为空时 */
    while (queue.length !== 0) {
        /** 记录当前队列长度 */
        const currentLevelQueueLength = queue.length
        /** 当前层队列 */
        const currentLevelOrder: number[] = []
        /** 遍历当前层节点，以此将节点出列，并将下层节点入列 */
        for (let i = 0; i < currentLevelQueueLength; i++) {
            const outQueueTreeNode = queue.shift()
            if (outQueueTreeNode.left) {
                queue.push(outQueueTreeNode.left)
                currentLevelOrder.push(outQueueTreeNode.left.val)
            }
            if (outQueueTreeNode.right) {
                queue.push(outQueueTreeNode.right)
                currentLevelOrder.push(outQueueTreeNode.right.val)
            }
        }
        /** 将当前节点队列加入层序遍历 */
        currentLevelOrder.length && levelOrder.push(currentLevelOrder)
    }
    return levelOrder
};