# FourSum.java

Below is the syntax highlighted version of FourSum.java from §1.4 Analysis of Algorithms.

/******************************************************************************
*  Compilation:  javac FourSum.java
*  Execution:    java FourSum < input.txt
*  Dependencies: StdIn.java StdOut.java
*
*  A program with N^4 running time. Read in N long integers
*  and counts the number of 4-tuples that sum to exactly 0.
*
*  Limitations
*  -----------
*     - we ignore integer overflow
*
******************************************************************************/

public class FourSum {

// print distinct 4-tuples (i, j, k, l) such that a[i] + a[j] + a[k] + a[l] = 0
public static int printAll(long[] a) {
int N = a.length;
int cnt = 0;
for (int i = 0; i < N; i++) {
for (int j = i+1; j < N; j++) {
for (int k = j+1; k < N; k++) {
for (int l = k+1; l < N; l++) {
if (a[i] + a[j] + a[k] + a[l] == 0) {
StdOut.println(a[i] + " " + a[j] + " " + a[k] + " " + a[l]);
}
}
}
}
}
return cnt;
}

// return number of distinct 4-tuples (i, j, k, l) such that a[i] + a[j] + a[k] + a[l] = 0
public static int count(long[] a) {
int N = a.length;
int cnt = 0;
for (int i = 0; i < N; i++) {
for (int j = i+1; j < N; j++) {
for (int k = j+1; k < N; k++) {
for (int l = k+1; l < N; l++) {
if (a[i] + a[j] + a[k] + a[l] == 0) {
cnt++;
}
}
}
}
}
return cnt;
}

public static void main(String[] args)  {
long[] a = new long[N];
for (int i = 0; i < N; i++) {