# 654. Maximum Binary Tree

You are given an integer array `nums` with no duplicates. A maximum binary tree can be built recursively from `nums` using the following algorithm:

1. Create a root node whose value is the maximum value in `nums`.
2. Recursively build the left subtree on the subarray prefix to the left of the maximum value.
3. Recursively build the right subtree on the subarray suffix to the right of the maximum value.

Return  the maximum binary tree built from`nums`.

Example 1:

``````Input: nums = [3,2,1,6,0,5]
Output: [6,3,5,null,2,0,null,null,1]
Explanation: The recursive calls are as follow:
- The largest value in [3,2,1,6,0,5] is 6. Left prefix is [3,2,1] and right suffix is [0,5].
- The largest value in [3,2,1] is 3. Left prefix is [] and right suffix is [2,1].
- Empty array, so no child.
- The largest value in [2,1] is 2. Left prefix is [] and right suffix is [1].
- Empty array, so no child.
- Only one element, so child is a node with value 1.
- The largest value in [0,5] is 5. Left prefix is [0] and right suffix is [].
- Only one element, so child is a node with value 0.
- Empty array, so no child.
``````

Example 2:

``````Input: nums = [3,2,1]
Output: [3,null,2,null,1]
``````

Constraints:

• `1 <= nums.length <= 1000`
• `0 <= nums[i] <= 1000`
• All integers in `nums` are unique.

``````class Solution {
public:
TreeNode* constructMaximumBinaryTree(vector<int>& nums) {
if (nums.empty()) return NULL;
int mx = INT_MIN, mx_idx = 0;
for (int i = 0; i < nums.size(); ++i) {
if (mx < nums[i]) {
mx = nums[i];
mx_idx = i;
}
}
TreeNode *node = new TreeNode(mx);
vector<int> leftArr = vector<int>(nums.begin(), nums.begin() + mx_idx);
vector<int> rightArr = vector<int>(nums.begin() + mx_idx + 1, nums.end());
node->left = constructMaximumBinaryTree(leftArr);
node->right = constructMaximumBinaryTree(rightArr);
return node;
}
};
``````

``````class Solution {
public:
TreeNode* constructMaximumBinaryTree(vector<int>& nums) {
if (nums.empty()) return NULL;
return helper(nums, 0, nums.size() - 1);
}
TreeNode* helper(vector<int>& nums, int left, int right) {
if (left > right) return NULL;
int mid = left;
for (int i = left + 1; i <= right; ++i) {
if (nums[i] > nums[mid]) {
mid = i;
}
}
TreeNode *node = new TreeNode(nums[mid]);
node->left = helper(nums, left, mid - 1);
node->right = helper(nums, mid + 1, right);
return node;
}
};
``````

``````class Solution {
public:
TreeNode* constructMaximumBinaryTree(vector<int>& nums) {
vector<TreeNode*> v;
for (int num : nums) {
TreeNode *cur = new TreeNode(num);
while (!v.empty() && v.back()->val < num) {
cur->left = v.back();
v.pop_back();
}
if (!v.empty()) {
v.back()->right = cur;
}
v.push_back(cur);
}
return v.front();
}
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/654

Maximum Binary Tree II

https://leetcode.com/problems/maximum-binary-tree/

https://leetcode.com/problems/maximum-binary-tree/discuss/106146/C%2B%2B-O(N)-solution

https://leetcode.com/problems/maximum-binary-tree/discuss/106194/javac-simple-recursive-method

https://leetcode.com/problems/maximum-binary-tree/discuss/106147/C%2B%2B-8-lines-O(n-log-n)-map-plus-stack-with-binary-search

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