LeetCode 654 Maximum Binary Tree(递归)

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本文介绍了一种基于给定整数数组构建最大二叉树的方法。通过递归找到数组中的最大值作为根节点，并以此划分左右子数组继续构建子树。文章提供了完整的Java代码实现，适用于LeetCode上的相关题目。

Given an integer array with no duplicates. A maximum tree building on this array is defined as follow:

The root is the maximum number in the array.
The left subtree is the maximum tree constructed from left part subarray divided by the maximum number.
The right subtree is the maximum tree constructed from right part subarray divided by the maximum number.

Construct the maximum tree by the given array and output the root node of this tree.

Example 1:

Input: [3,2,1,6,0,5]
Output: return the tree root node representing the following tree:

      6
    /   \
   3     5
    \    / 
     2  0   
       \
        1

Note:

The size of the given array will be in the range [1,1000].

题目链接：https://leetcode.com/problems/maximum-binary-tree/description/

题目分析：基础的递归构造，击败97%

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    public TreeNode constructHelper(int[] nums, int l, int r) {
        if (l > r) {
            return null;
        }
        if (l == r) {
            return new TreeNode(nums[l]);
        }
        int ma_idx = l;
        for (int i = l + 1; i <= r; i ++) {
            if (nums[i] > nums[ma_idx]) {
                ma_idx = i;
            }  
        }
        TreeNode root = new TreeNode(nums[ma_idx]);
        root.left = constructHelper(nums, l, ma_idx - 1);
        root.right = constructHelper(nums, ma_idx + 1, r);
        return root;
    }
    
    public TreeNode constructMaximumBinaryTree(int[] nums) {
        return constructHelper(nums, 0, nums.length - 1);
    }
}