【二叉树】654.最大二叉树

题目

法1：递归

Python

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def constructMaximumBinaryTree(self, nums: List[int]) -> Optional[TreeNode]:
        if len(nums) == 0:
            return None
        if len(nums) == 1:
            return TreeNode(nums[0])

        max_val = max(nums)
        max_idx = nums.index(max_val)
        root = TreeNode(max_val)
        root.left = self.constructMaximumBinaryTree(nums[:max_idx])
        root.right = self.constructMaximumBinaryTree(nums[max_idx+1:])

        return root

Java

class Solution {
    public TreeNode constructMaximumBinaryTree(int[] nums) {
        TreeNode root = build(nums, 0, nums.length - 1);
        return root;
    }

    public TreeNode build(int[] nums, int start, int end) {
        if (start > end) {
            return null;
        }
        if (start == end) {
            return new TreeNode(nums[start]);
        }
        int maxInx = start;
        int maxVal = nums[start];
        for (int i = start; i <= end; ++i) {
            if (nums[i] > nums[maxInx]) {
                maxInx = i;
                maxVal = nums[i];
            }
        }
        TreeNode root = new TreeNode(maxVal);
        root.left = build(nums, start, maxInx - 1);
        root.right = build(nums, maxInx + 1, end);

        return root;
    }
}