LeetCode -- Convert SortedList To BST

LeetCode -- Convert SortedList To BST

 


Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.


就是把鏈表轉化為二叉查找樹


思路：
使用分治策略
1.把鏈表節點遍歷，存在nodes集合中
2.用[0, length/2)節點創建左子樹，用(length/2,n]節點創建右子樹，使用nodes[length/2]來創建當前節點。




實現代碼：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode SortedListToBST(ListNode head) {
        if(head == null){
		return null;
	}
	
	var nodes = new List();
	while(head != null){
		nodes.Add(head.val);
		head = head.next;
	}
	
	TreeNode root = null;
	BuildTree(nodes,ref root);
	
	return root;
}


private void BuildTree(IList values,ref TreeNode n){
	if(values.Count == 0){
		return;
	}


	var mid = (int)(values.Count/2);
	var self = values[mid];
	n = new TreeNode(self);
	
	var leftVals=  values.Where(x=>x < self);
	var rightVals = values.Where(x=>x > self);
	
	if(leftVals.Any()){
		BuildTree(leftVals.ToList() ,ref n.left);
	}
	if(rightVals.Any()){
		BuildTree(rightVals.ToList(),ref n.right);
	}
}




}