FourSum.java

import java.util.ArrayList;
import java.util.Arrays;

/**
 * Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d =
 * target?
 *
 * <p>Find all unique quadruplets in the array which gives the sum of target.
 *
 * <p>Note:
 *
 * <p>Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a <= b <= c <= d) The
 * solution set must not contain duplicate quadruplets.
 *
 * <p>For example, given array S = {1 0 -1 0 -2 2}, and target = 0.
 *
 * <p>A solution set is: (-1, 0, 0, 1) (-2, -1, 1, 2) (-2, 0, 0, 2)
 */
public class FourSum {
  public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
    ArrayList<ArrayList<Integer>> ret = new ArrayList<ArrayList<Integer>>();
    Arrays.sort(num);
    int length = num.length;
    for (int i = 0; i < length - 3; i++) {
      if (i > 0 && num[i] == num[i - 1]) continue;
      for (int j = i + 1; j < length - 2; j++) {
        if (j > i + 1 && num[j] == num[j - 1]) continue;
        int l = j + 1;
        int r = length - 1;
        while (l < r) {
          int delta = num[i] + num[j] + num[l] + num[r] - target;
          if (delta == 0) {
            if (l > j + 1 && num[l] == num[l - 1]) {
              l++;
              continue;
            }
            if (r < length - 1 && num[r] == num[r + 1]) {
              r--;
              continue;
            }
            ArrayList<Integer> item = new ArrayList<Integer>();
            item.add(num[i]);
            item.add(num[j]);
            item.add(num[l]);
            item.add(num[r]);
            ret.add(item);
            l++;
            r--;
          } else if (delta < 0) {
            l++;
          } else {
            r--;
          }
        }
      }
    }
    return ret;
  }
}