[LintCode] 177 Convert Sorted Array to Binary Search Tree With Minimal Height

Description
Given a sorted (increasing order) array, Convert it to create a binary tree with minimal height.


Notice
There may exist multiple valid solutions, return any of them.


Example
Given [1,2,3,4,5,6,7], return
      4
     /   \
   2    6
  /  \  /  \
1   3 5  7



思路
用二分法。每次以A[mid]的值做一个node。node.left和right接着用二分去做。


Code

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param A: an integer array
     * @return: a tree node
     */
    public TreeNode sortedArrayToBST(int[] A) {  
        // write your code here
        if (A == null || A.length == 0) {
            return null;
        }
        
        return buildBST(A, 0, A.length - 1);
    }  
    
    private TreeNode buildBST(int[] A, int start, int end) {
        if (start > end) {
            return null;
        }
        
        int mid = start + (end - start) / 2;
        TreeNode root = new TreeNode(A[mid]);
        root.left = buildBST(A, start, mid - 1);
        root.right = buildBST(A, mid + 1, end);
        
        return root;
    }
}