LeetCode-654. Maximum Binary Tree

最新推荐文章于 2020-02-28 11:53:05 发布

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#LeetCode #二叉树练习

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本文介绍了一种构建最大二叉树的方法，通过不断寻找数组中的最大值作为根节点，并递归地构造左右子树。文章提供了详细的算法实现过程及示例。

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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

Example 2:

Input: [3,2,1,6,0,5]

Output: [6,3,5,null,2,0,null,null,1]

Note:

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

function:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

/*
 *   找出最大的数字即根，再不断向左向右遍历找出左孩子与右孩子中各自最大的孩子，以此循环。
 */

class Solution {
    public TreeNode constructMaximumBinaryTree(int[] nums) {
        return construct(nums,0,nums.length);
    }
    
    public TreeNode construct(int[] nums,int l,int r){
        if(l==r) return null;
        
        int max_i=maxIndex(nums,l,r);
        TreeNode t= new TreeNode(nums[max_i]);
        t.left=construct(nums,l,max_i);
        t.right=construct(nums,max_i+1,r);
        
        return t;
        
    }
    
    public int maxIndex(int[] nums,int l,int r){
        int max=l;
        for(int i=l+1;i<r;i++){
            if(nums[max]<nums[i]) max=i;
        }
        return max;
    }
}