题解 | #从上往下打印二叉树#

/*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)
 *     }
 * }
 */

/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 * 
 * @param root TreeNode类 
 * @return int整型一维数组
 */
export function PrintFromTopToBottom(root: TreeNode): number[] {
    // write code here
    let queue = root ? [root] : [], len = queue.length, res = []
    while(len > 0) {
        let node = queue.shift()
        len--
        res.push(node.val)
        if(node.left) {
            queue.push(node.left)
            len++
        } 
        if(node.right) {
            queue.push(node.right)
            len++
        }
    }
    return res
}