算法第十七周作业01

Description

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

Solution

获取左子树的节点数number

如果左子树节点数刚好为k-1，说明该节点为第k个，否则向左子树找或者向右子树找。

Code

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int kthSmallest(TreeNode root, int k) {
int number = number(root.left);

if (number == k - 1) {
// 如果左子树节点数刚好为k-1，说明该节点为第k个
return root.val;
} else if (number >= k) {
// 第k个在左子树
return kthSmallest(root.left, k);
} else {
// 第k个在右子树
return kthSmallest(root.right, k - 1 - number);
}
}

// 获取该节点的所有叶子数
public int number(TreeNode node) {
if (node == null) {
return 0;
} else {
return number(node.left) + number(node.right) + 1;
}
}
}