Talk is cheap.

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func kthNode(root *TreeNode, k int) *TreeNode {
    var stack []*TreeNode
    cnt := 0
    node := root
    for len(stack) != 0 || root != nil {
        for root != nil {
            stack = append(stack, root)
            root = root.Left
        }
        node, stack = stack[len(stack)-1], stack[:len(stack)-1]
        cnt += 1
        if cnt == k { return node }
        root = node.Right
    }
    return nil
}