Solution.cpp

class Node {
public:
    Node* left;
    Node* right;
    int l;
    int r;
    int mid;
    int v;
    int add;

    Node(int l, int r) {
        this->l = l;
        this->r = r;
        this->mid = (l + r) >> 1;
        this->left = this->right = nullptr;
        v = add = 0;
    }
};

class SegmentTree {
private:
    Node* root;

public:
    SegmentTree() {
        root = new Node(1, 1e9);
    }

    void modify(int l, int r, int v) {
        modify(l, r, v, root);
    }

    void modify(int l, int r,int v, Node* node) {
        if (l > r) return;
        if (node->l >= l && node->r <= r)
        {
            node->v = v;
            node->add = v;
            return;
        }
        pushdown(node);
        if (l <= node->mid) modify(l, r, v, node->left);
        if (r > node->mid) modify(l, r, v, node->right);
        pushup(node);
    }

    int query(int l, int r) {
        return query(l, r, root); 
    }

    int query(int l, int r, Node* node) {
        if (l > r) return 0;
        if (node->l >= l && node-> r <= r) return node->v;
        pushdown(node);
        int v = 0;
        if (l <= node->mid) v = max(v, query(l, r, node->left));
        if (r > node->mid) v = max(v, query(l, r, node->right));
        return v; 
    }

    void pushup(Node* node) {
        node->v = max(node->left->v, node->right->v);
    }

    void pushdown(Node* node) {
        if (!node->left) node->left = new Node(node->l, node->mid);
        if (!node->right) node->right = new Node(node->mid + 1, node->r);
        if (node->add)
        {
            Node* left = node->left;
            Node* right = node->right;
            left->v = node->add;
            right->v = node->add;
            left->add = node->add;
            right->add = node->add;
            node->add = 0;
        }
    }
};

class Solution {
public:
    vector<int> fallingSquares(vector<vector<int>>& positions) {
        vector<int> ans;
        SegmentTree* tree = new SegmentTree();
        int mx = 0;
        for (auto& p : positions)
        {
            int l = p[0], w = p[1], r = l + w - 1;
            int h = tree->query(l, r) + w;
            mx = max(mx, h);
            ans.push_back(mx);
            tree->modify(l, r, h);
        }
        return ans;
    }
};