题目：

题解：

func reverse(a []int) {
    for i, n := 0, len(a); i < n/2; i++ {
        a[i], a[n-1-i] = a[n-1-i], a[i]
    }
}

func postorderTraversal(root *TreeNode) (res []int) {
    addPath := func(node *TreeNode) {
        resSize := len(res)
        for ; node != nil; node = node.Right {
            res = append(res, node.Val)
        }
        reverse(res[resSize:])
    }

    p1 := root
    for p1 != nil {
        if p2 := p1.Left; p2 != nil {
            for p2.Right != nil && p2.Right != p1 {
                p2 = p2.Right
            }
            if p2.Right == nil {
                p2.Right = p1
                p1 = p1.Left
                continue
            }
            p2.Right = nil
            addPath(p1.Left)
        }
        p1 = p1.Right
    }
    addPath(root)
    return
}