Leetcode-Day14

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从零开始Leetcode – Day 14

Convert Sorted List to Binary Search Tree

https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree/

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/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {TreeNode}
 */
var sortedListToBST = function(head) {
    if(!head){
        return null;
    }
    var cur = head;
    var count = 0;
    while(cur !== null){
        cur = cur.next;
        count++;
    }
    var list = [];
    list.push(head);
    return helper(list, 0, count-1);
};

var helper = function(list, l, r){
    if(l>r){
        return null;
    }
    var m = Math.floor((l+r)/2);
    // left 
    var left = helper(list, l, m-1);
    
    var root = new TreeNode(list[0].val);
    root.left = left;
    list[0] = list[0].next;
    
    // right
    root.right = helper(list, m+1, r);
    return root;
}