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Maximum Binary Tree

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Description

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

Solution

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* constructMaximumBinaryTree(vector<int>& nums) {
        TreeNode* ret = NULL;
        helper(nums, ret, 0, nums.size() - 1);
        return ret;
    }
    
    void helper(vector<int>& nums, TreeNode*& ret, int start, int end) {
        if (start > end) return;
        int maxium = INT_MIN;
        int target = -1;
        for (int i = start; i <= end; ++i) {
            if (nums[i] > maxium) {
                maxium = max(nums[i], maxium);
                target = i;   
            }
        }
        
        ret = new TreeNode(maxium);
        helper(nums, ret->left, start, target - 1);
        helper(nums, ret->right, target + 1, end);
    }
};

Segment Tree

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
struct SegNode{
    int leftBound;
    int rightBound;
    pair<int, int> maxium;
};

class Solution {
public:
    TreeNode* constructMaximumBinaryTree(vector<int>& nums) {
        TreeNode* ret = NULL;
        if (nums.size() == 0) return ret;
        vector<SegNode> seg(nums.size() * 4);
        build(seg, nums, 0, nums.size() - 1, 0);
        
        helper(nums, ret, 0, nums.size() - 1, seg);
        return ret;
    }
    
    pair<int, int> build(vector<SegNode>& seg, vector<int>& nums, int start, int end, int index) {
        if (start > end) return make_pair(INT_MIN, -1);
        seg[index].leftBound = start;
        seg[index].rightBound = end;
        
        if (start == end) {
            seg[index].maxium = make_pair(nums[start], start);
            return seg[index].maxium;
        }
        int mid = (start + end) >> 1;
        
        auto l = build(seg, nums, start, mid, index * 2 + 1);
        auto r = build(seg, nums, mid + 1, end, index * 2 + 2);
        seg[index].maxium = l.first > r.first ? l : r;
        return seg[index].maxium;
    }
    
    pair<int, int> query(int left, int right, vector<SegNode>& seg, int index) {
        if (left > right) return make_pair(INT_MIN, -1);
        int leftBound = seg[index].leftBound;
        int rightBound = seg[index].rightBound;
        int mid = (leftBound + rightBound) >> 1;
        if (left == leftBound && right == rightBound) return seg[index].maxium;
        if (left > mid) return query(left, right, seg, index * 2 + 2);
        if (right <= mid) return query(left, right, seg, index * 2 + 1);
        
        auto l = query(left, mid, seg, index * 2 + 1);
        auto r = query(mid + 1, right, seg, index * 2 + 2);
        return l.first > r.first ? l : r;
    }
    
    void helper(vector<int>& nums, TreeNode*& ret, int start, int end, vector<SegNode>& seg) {
        if (start > end) return;
        auto target = query(start, end, seg, 0);
        
        ret = new TreeNode(target.first);
        ret->left = NULL;
        ret->right = NULL;
        helper(nums, ret->left, start, target.second - 1, seg);
        helper(nums, ret->right, target.second + 1, end, seg);
    }
};