https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

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/**
 * 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) {
    let res = []
    bfs(root, res, 0)
    return res.reverse()
};

const bfs = (root, res, level) => {
    if (!root) {
        return
    }
    bfs(root.left, res, level + 1)
    bfs(root.right, res, level + 1)
    if (!res[level]) {
        res[level] = [root.val]
    } else {
        res[level].push(root.val)
    }
}