二叉树的层次遍历

/**
 * 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[][] {
    //广度优先遍历
    //队列实现层次遍历
    if(root  === null) return []
    let queue = []
    let res = []
    
    queue.push(root)
 
    while(queue.length>0){
        
        let length = queue.length
        let queue_val = []
        for(let i =0;i<length;i++){
            const node = queue.shift()
            //将左右字节点都给 push 到队列当中去
            queue_val.push(node.val)
            node.left && queue.push(node.left)
            node.right && queue.push(node.right)
            
        }
        res.push(queue_val)
    }
    return res
};