【LeetCode】Convert Sorted Array to Binary Search Tree && Convert Sorted List to Binary Search Tree

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1、Convert Sorted Array to Binary Search Tree
Total Accepted: 8749 Total Submissions: 27493 My Submissions
Given an array where elements are sorted in ascending order, convert it to a height balanced BST.
2、Convert Sorted List to Binary Search Tree
Total Accepted: 7649 Total Submissions: 28780 My Submissions
Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
升序数组转为二叉搜索树。
其实2本质上是1的变形，尽管2是链表，但是可以转成数组，扫描一遍链表即可完成。
针对1来说，每次搜索，需要搜索中间那个数字，作为root节点。int mid = (low + high) >> 1。递归深入，即可求得结果。
针对2来说，有两种办法。1)是扫描一遍，将链表转为数字，然后调用1的方法即可解决。2)扫描链表，每次用快慢指针来找到中间节点。这个要比方法1快30ms左右。

1、Java AC

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode sortedArrayToBST(int[] num) {if (num == null || num.length == 0) {return null;}int len = num.length;return createBSTree(num, 0, len - 1);}private TreeNode createBSTree(int[] num, int low, int high) {if (low > high) {return null;}if (low == high) {return new TreeNode(num[low]);}int mid = (low + high) >> 1;TreeNode root = new TreeNode(num[mid]);root.left = createBSTree(num, low, mid - 1);root.right = createBSTree(num, mid + 1, high);return root;}}

2、1)Java AC

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; next = null; } * } *//** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode sortedListToBST(ListNode head) {        if(head == null){            return null;        }        int len = 0;        List<Integer> list = new ArrayList<Integer>();        while(head != null){            list.add(head.val);            head = head.next;            len++;        }        return createBSTree(list, 0, len - 1);    }private TreeNode createBSTree(List<Integer> list, int low, int high) {if (low > high) {return null;}if (low == high) {return new TreeNode(list.get(low));}int mid = (low + high) >> 1;TreeNode root = new TreeNode(list.get(mid));root.left = createBSTree(list, low, mid - 1);root.right = createBSTree(list, mid + 1, high);return root;}}

2、2)Java AC

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; next = null; } * } *//** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode sortedListToBST(ListNode head) {        if(head == null){            return null;        }        return createBSTree(head, null);    }private TreeNode createBSTree(ListNode start, ListNode end) {if(start == end){    return null;}ListNode fastNode = start;ListNode slowNode = start;while(fastNode != end && fastNode.next != end){    slowNode = slowNode.next;    fastNode = fastNode.next.next;}TreeNode root = new TreeNode(slowNode.val);root.left = createBSTree(start, slowNode);root.right = createBSTree(slowNode.next, end);return root;}}

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