將有序數組轉換爲二叉搜索樹

LeetCode108 解題思路

二叉樹的中序遍歷是升序序列,數組有序,能夠確保是中序遍歷,但中序遍歷沒法徹底肯定一棵二叉樹。即結果不惟一。
每次以數組的中間數字做爲二叉樹的「根」結點,左右子樹的元素分別位於「根」結點的兩遍,依次遞歸結束java

代碼

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        return helper(nums,0,nums.length-1);
    }
    public TreeNode helper(int[] nums,int left,int right){
        if(left>right){
            return null;
        }
        int mid=(left+right)/2;
        TreeNode root=new TreeNode(nums[mid]);
        root.left=helper(nums,left,mid-1);
        root.right=helper(nums,mid+1,right);
        return root;
    }
}
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