Kth smallest element in a BST

Question

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

Solution

因為是 BST 的關係，用 in-order 可以保證遞增順序，所以就很簡單的把它存在 array 裡面再 return 就可以了。這邊想記錄的是在 BST 裡面做 binary search

public int kthSmallest(TreeNode root, int k) {
        int count = countNodes(root.left);
        if (k <= count) {
            return kthSmallest(root.left, k);
        } else if (k > count + 1) {
            return kthSmallest(root.right, k-1-count); // 1 is counted as current node
        }

        return root.val;
    }

    public int countNodes(TreeNode n) {
        if (n == null) return 0;

        return 1 + countNodes(n.left) + countNodes(n.right);
    }