NC45 实现二叉树先序，中序和后序遍历

https://www.nowcoder.com/practice/a9fec6c46a684ad5a3abd4e365a9d362

这题可以使用非递归的形式，但是为了快速解题这里就用传统的方式了

/*
 * type TreeNode struct {
 *   Val int
 *   Left *TreeNode
 *   Right *TreeNode
 * }
 */

func preorderTraversal(root *TreeNode, result *[]int) {
    if root == nil {
        return
    }
    *result = append(*result, root.Val)
    preorderTraversal(root.Left, result)
    preorderTraversal(root.Right, result)
}

func inorderTraversal(root *TreeNode, result *[]int) {
    if root == nil {
        return
    }
    inorderTraversal(root.Left, result)
    *result = append(*result, root.Val)
    inorderTraversal(root.Right, result)
}

func postorderTraversal(root *TreeNode, result *[]int) {
    if root == nil {
        return
    }
    postorderTraversal(root.Left, result)
    postorderTraversal(root.Right, result)
    *result = append(*result, root.Val)
}

/**
 * 
 * @param root TreeNode类 the root of binary tree
 * @return int整型二维数组
*/
func threeOrders( root *TreeNode ) [][]int {
    var parr, inarr, poarr = make([]int, 0), make([]int, 0), make([]int, 0)
    preorderTraversal(root, &parr)
    inorderTraversal(root, &inarr)
    postorderTraversal(root, &poarr)
    return [][]int {parr, inarr, poarr}
}