0230-kth-smallest-element-in-a-bst.go

func kthSmallest(root *TreeNode, k int) int {
	stack := make([]*TreeNode, 0, k)

	for {
		for root != nil {
			stack = append(stack, root)
			root = root.Left
		}

		root = stack[len(stack) - 1]
		stack = stack[:len(stack) - 1]

		k--
		if k == 0 {
			return root.Val
		}

		root = root.Right
	}

	return -1
}