[LeetCode] Problem 654 - Maximum Binary Tree

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

  1. The root is the maximum number in the array.
  2. The left subtree is the maximum tree constructed from left part subarray divided by the maximum number.
  3. 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

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

Output: return the tree root node representing the following tree:

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2
3
4
5
6
7
   6
 /   \
3     5
 \    / 
  2  0   
    \
     1

Note

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

Code

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public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
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public TreeNode constructMaximumBinaryTree(int[] nums) {
    return construct(nums, 0, nums.length - 1);
}

private TreeNode construct(int[] nums, int start, int end) {
    if (start > end)
        return null;

    int max = nums[start];
    int idx = start;

    for (int i = start + 1; i <= end; i++) {
        if (nums[i] > max) {
            max = nums[i];
            idx = i;
        }
    }

    TreeNode root = new TreeNode(nums[idx]);
    root.left = construct(nums, start, idx - 1);
    root.right = construct(nums, idx + 1, end);
    return root;
}