二叉树的层次遍历 II

给定二叉树 [3,9,20,null,null,15,7]

    3
   / \
  9  20
    /  \
   15   7
   
返回其自底向上的层次遍历为

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

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrderBottom = function(root) {
    if(!root) return [];
    var queue = [root];
    var target = [];
    while(queue.length){
        let tmp = [];
        let levelSize = queue.length;
        for(let i=0;i<levelSize;++i){
            let cur = queue.shift();
            tmp.push(cur.val);
            if(cur.left) queue.push(cur.left);
            if(cur.right) queue.push(cur.right);
        }
        target.unshift(tmp);
    }
    return target;
};

https://github.com/WindrunnerMax/EveryDay

https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/solution/er-cha-shu-de-ceng-ci-bian-li-ii-by-leetcode-solut/