LeetCode刷題筆錄Convert Sorted Array to Balanced Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

這題可以一眼看出是要用遞歸，不會是叫你寫一個BST的插入。每次選出sorted array中間的那個元素作為目前的node,這個node的左兒子是左半邊數組的中間元素，右兒子是右半邊數組的中間元素。這樣就有Subproblem了。

coding的時候注意subproblem的範圍是(low, mid - 1)和(mid + 1, high)，因為mid元素已經用過了。

發現自己遞歸還是要多練練，想到了這是遞歸也用了很久才寫出代碼，sigh

/**
 * 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) {
        return helper(num, 0, num.length - 1);
    }
    public TreeNode helper(int[] num, int low, int high){
        if(low > high)
            return null;
        int mid = (low + high) / 2;
        
        TreeNode node = new TreeNode(num[mid]);
        node.left = helper(num, low, mid - 1);
        node.right = helper(num, mid + 1, high);
        return node;
            
    }
}