1115.print-foobar-alternately

Statement

简体中文English

Metadata

• Link: 交替打印 FooBar
• Difficulty: Medium
• Tag: 多线程

给你一个类：

class FooBar {
  public void foo() {
    for (int i = 0; i < n; i++) {
      print("foo");
    }
  }
  public void bar() {
    for (int i = 0; i < n; i++) {
      print("bar");
    }
  }
}

两个不同的线程将会共用一个 FooBar 实例：

• 线程 A 将会调用 foo() 方法，而
• 线程 B 将会调用 bar() 方法

请设计修改程序，以确保 "foobar" 被输出 n 次。

 

示例 1：

输入：n = 1
输出："foobar"
解释：这里有两个线程被异步启动。其中一个调用 foo() 方法, 另一个调用 bar() 方法，"foobar" 将被输出一次。

示例 2：

输入：n = 2
输出："foobarfoobar"
解释："foobar" 将被输出两次。

 

提示：

• 1 <= n <= 1000

Metadata

• Link: Print FooBar Alternately
• Difficulty: Medium
• Tag: Concurrency

Suppose you are given the following code:

class FooBar {
  public void foo() {
    for (int i = 0; i < n; i++) {
      print("foo");
    }
  }
  public void bar() {
    for (int i = 0; i < n; i++) {
      print("bar");
    }
  }
}

The same instance of FooBar will be passed to two different threads:

• thread A will call foo(), while
• thread B will call bar().

Modify the given program to output "foobar" n times.

 

Example 1:

Input: n = 1
Output: "foobar"
Explanation: There are two threads being fired asynchronously. One of them calls foo(), while the other calls bar().
"foobar" is being output 1 time.

Example 2:

Input: n = 2
Output: "foobarfoobar"
Explanation: "foobar" is being output 2 times.

 

Constraints:

• 1 <= n <= 1000

Solution

C++

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define endl "\n"
#define fi first
#define se second
#define all(x) begin(x), end(x)
#define rall rbegin(a), rend(a)
#define bitcnt(x) (__builtin_popcountll(x))
#define complete_unique(a) a.erase(unique(begin(a), end(a)), end(a))
#define mst(x, a) memset(x, a, sizeof(x))
#define MP make_pair

using ll = long long;
using ull = unsigned long long;
using db = double;
using ld = long double;
using VLL = std::vector<ll>;
using VI = std::vector<int>;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;

using namespace __gnu_pbds;
using namespace std;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

template <typename T, typename S>
inline bool chmax(T &a, const S &b) {
    return a < b ? a = b, 1 : 0;
}

template <typename T, typename S>
inline bool chmin(T &a, const S &b) {
    return a > b ? a = b, 1 : 0;
}

#ifdef LOCAL
#include <debug.hpp>
#else
#define dbg(...)
#endif
// head

class FooBar {
private:
    int n;

public:
    FooBar(int n) {
        this->n = n;
    }

    void foo(function<void()> printFoo) {
        for (int i = 0; i < n; i++) {
            while (order_.load() % 2 != 0) {
                std::this_thread::yield();
            }

            // printFoo() outputs "foo". Do not change or remove this line.
            printFoo();

            order_++;
        }
    }

    void bar(function<void()> printBar) {
        for (int i = 0; i < n; i++) {
            while (order_.load() % 2 != 1) {
                std::this_thread::yield();
            }

            // printBar() outputs "bar". Do not change or remove this line.
            printBar();

            order_++;
        }
    }

private:
    std::atomic<int> order_{0};
};

#ifdef LOCAL

int main() {
    return 0;
}

#endif

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最后更新: October 11, 2023