[Day39]-[二叉树] 二叉搜索树中第 K 小的元素

输入：root = [3,1,4,null,2], k = 1输出：1

输入：root = [5,3,6,2,4,null,null,1], k = 3输出：3

func TestKthSmallest(t *testing.T) {	array := []int{1, 2, 3, 4, 5}	var buildTree func(left, right int) *TreeNode	buildTree = func(left, right int) *TreeNode { // 构建一颗二叉树		if left > right {			return nil		}
		mid := (left + right) >> 1		return &TreeNode{			array[mid],			buildTree(left, mid-1),			buildTree(mid+1, right),		}	}	root := buildTree(0, len(array)-1)	var list []int	var k = 3	var dfs func(root *TreeNode) int	dfs = func(root *TreeNode) int {		if root != nil {			dfs(root.Left)			list = append(list, root.Val)			if len(list) == k {				return root.Val			}			dfs(root.Right)		}		return -1	}
	t.Log(dfs(root))}