Блог пользователя tgbaodeeptry

Автор tgbaodeeptry, история, 3 года назад, По-английски

Hello guys, after learning Segment Tree, I approach to learn segment tree with LAZY PROPAGATION I tried to implemented it by do this task increase V to all elements in range a to b and get sum of elements in this range. This is my code:

#include <bits/stdc++.h>
using namespace std;

struct Node {
    int val;
    int lazy;
};

struct Segment {
    int n;
    Node nodes[400001];

    Segment (int n) {
        this->n = n;

        for (int i = 0; i <= 4*n; i++) 
            nodes[i] = {0, 0};
    }

    void down(int id) {
        int t = nodes[id].lazy;

        nodes[id << 1].val += t;
        nodes[id << 1].lazy += t;
        nodes[id << 1 | 1].val += t;
        nodes[id << 1 | 1].lazy += t;

        nodes[id].lazy = 0;
    }

    void update(int id, int l, int r, int u, int v, int val) {
        if (l > v || r < u) 
            return;

        if (u <= l && r <= v) {
            nodes[id].val += val;
            nodes[id].lazy += val;
            return;
        }

        down(id);
        
        int m = (l + r) >> 1;
        update(id << 1, l, m, u, v, val);
        update(id << 1 | 1, m + 1, r, u, v, val);

        nodes[id].val = nodes[id << 1].val + nodes[id << 1 | 1].val;
    }

    int get(int id, int l, int r, int u, int v) {
        if (l > v || r < u) {
            return 0;
        }

        if (u <= l && r <= v) 
            return nodes[id].val;

        down(id);

        int m = (l + r) >> 1;
        return get(id << 1, l, m, u, v) + get(id << 1 | 1, m + 1, r, u, v);
    }
};

int main() {
    Segment seg(10);
    seg.update(1, 1, 10, 1, 5, 10);

    cout << seg.get(1, 1, 10, 1, 5) << endl;
}

And the answer after running is 10 (but correct answer is 50). Did I implement wrongly?

Thanks so much, guys :D

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»
3 года назад, # |
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Have you tried the following:

  • Write a very naive and simple solution to the problem (maybe exponential complexity) (if you can, you may also just copy someone else's solution)
  • Write a program to generate thousands of small (!) test cases
  • Using those two, find a test case where your program gives the wrong answer
  • Use print statements or a debugger to see where exactly your program does the wrong thing.

98% of WAs and REs can be resolved this way. People here don't have the time to delve into every code posted here, it's much harder to debug somebody else's code and being able to debug your own code is a valuable skill. It is also a very routine process that can be learned much faster than problem solving and algorithms.

»
3 года назад, # |
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Line 36 should be nodes[id].val += (r - l + 1) * val;

There might be some other mistakes as well, didn't see the code carefully.