LeetCode_230 Kth Smallest Element in a BST

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

Note:

You may assume k is always valid, 1 ≤ k ≤ BST’s total elements.

Follow up:

What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

计算左子树的size

left+1 = K，则根节点即为第K个元素

left >=k, 则第K个元素在左子树中，

left +1

public int kthSmallest(TreeNode root, int k) {
if(root == null) return 0;
int leftSize = calSize(root.left);
if(leftSize +1==k){
return root.val;
}else if(leftSize >= k) {
return kthSmallest(root.left, k);
}else {
return kthSmallest(root.right, k-leftSize-1);
}
}
private int calSize(TreeNode root){
if(root == null) return 0;
return 1+ calSize(root.left)+calSize(root.right);
}