ThreeSum.h

#ifndef CH1_THREESUM_H
#define CH1_THREESUM_H

#include <vector>
#include <iostream>

using std::vector;
using std::cout;
using std::endl;

/**
 *  The {@code ThreeSum} class provides static methods for counting
 *  and printing the number of triples in an array of integers that sum to 0
 *  (ignoring integer overflow).
 *  <p>
 *  This implementation uses a triply nested loop and takes proportional to n^3,
 *  where n is the number of integers.
 *  <p>
 *  For additional documentation, see <a href="https://algs4.cs.princeton.edu/14analysis">Section 1.4</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
class ThreeSum {
public:
    // Do not instantiate.
    ThreeSum() = delete;

    /**
     * Prints to standard output the (i, j, k) with {@code i < j < k}
     * such that {@code a[i] + a[j] + a[k] == 0}.
     *
     * @param a the array of integers
     */
    static void printAll(vector<int> &a) {
        int n = a.size();
        for (int i = 0; i < n; i++) {
            for (int j = i + 1; j < n; j++) {
                for (int k = j + 1; k < n; k++) {
                    if (a[i] + a[j] + a[k] == 0) {
                        cout << a[i] << " " << a[j] << " " << -(a[i] + a[j]) << endl;
                    }
                }
            }
        }
    }

    /**
     * Returns the number of triples (i, j, k) with {@code i < j < k}
     * such that {@code a[i] + a[j] + a[k] == 0}.
     *
     * @param  a the array of integers
     * @return the number of triples (i, j, k) with {@code i < j < k}
     *         such that {@code a[i] + a[j] + a[k] == 0}
     */
    static int count(vector<int> &a) {
        int n = a.size();
        int count = 0;
        for (int i = 0; i < n; i++) {
            for (int j = i + 1; j < n; j++) {
                for (int k = j + 1; k < n; k++) {
                    if (a[i] + a[j] + a[k] == 0) {
                        count++;
                    }
                }
            }
        }
        return count;
    }
};

#endif //CH1_THREESUM_H