Java 递归分治法

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param num int整型一维数组 
     * @return TreeNode类
     */
    public TreeNode sortedArrayToBST (int[] num) {
        // write code here
        if(num == null){
            return null;
        }
        return helper(num, 0, num.length - 1);
    }

    private TreeNode helper(int[] num, int left, int right){
        if(left > right){
            return null;
        }
        int mid = left + (right - left + 1) / 2;
        TreeNode lNode = helper(num, left, mid - 1);
        TreeNode rNode = helper(num, mid + 1, right);
        TreeNode node = new TreeNode(num[mid]);
        if(lNode != null) node.left  = lNode;
        if(rNode != null) node.right = rNode;
        return node;
    }
}