lintcode-easy-Convert Sorted Array to Binary Search Tree with Minimal Height

Posted 2020-06-16

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

 

Example

Given [1,2,3,4,5,6,7], return

     4
   /     2     6
 / \    / 1   3  5   7

Note

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

 

思路比较直接，中间的数作为root，前一半作为左子树，后一半作为右子树

/**
 * 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;
        
        TreeNode root = getTree(A, 0, A.length - 1);
        
        return root;
    }  
    
    public TreeNode getTree(int[] A, int start, int end){
        if(start > end)
            return null;
            
        int mid = start + (end - start) / 2;
        
        TreeNode root = new TreeNode(A[mid]);
        
        TreeNode left = getTree(A, start, mid - 1);
        TreeNode right = getTree(A, mid + 1, end);
        
        root.left = left;
        root.right = right;
        
        return root;
    }
    
}