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
/** * 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; } }