题解 | #二叉树程序遍历 ii# 分层遍历+结果翻转

先分层遍历，再对结果进行翻转

func levelOrderBottom( root *TreeNode ) [][]int {
    // write code here
    if root == nil {
        return nil
    }

    ret := make([][]int, 0)
    stack := []*TreeNode{root}

//  先分层遍历
    for len(stack) >0 {
        size := len(stack)
        tmp := make([]int, 0, size)
        for i:=0; i< size; i++{
            node := stack[i]
            tmp = append(tmp, node.Val)

            if node.Left != nil {
                stack = append(stack, node.Left)
            }
            if node.Right != nil {
                stack = append(stack, node.Right)
            }
        }
        stack = stack[size:]

        ret = append(ret, tmp)
    }

//  最后对结果进行翻转
    if len(ret) > 0 {
        for i, j := 0, len(ret)-1; i<j; i,j = i+1, j-1 {
            ret[i], ret[j] = ret[j], ret[i]
        }
    }

    return ret
}