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FourSum.java
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80 lines (73 loc) · 2.33 KB
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package algorithm.lc;
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? Find all unique quadruplets in the array which
* gives the sum of target.
*
* Note:
*
* 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. For
* example, given array S = {1 0 -1 0 -2 2}, and target = 0.
*
* A solution set is: (-1, 0, 0, 1) (-2, -1, 1, 2) (-2, 0, 0, 2)
*
*/
// O(n^3) space, O(n^3) time
public class FourSum {
public class Solution {
// use two pointers inside a two-level nested loop
public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
// Start typing your Java solution below
// DO NOT write main() function
Arrays.sort(num);
ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
for (int i = 0; i < num.length - 3; ++i) {
if (i > 0 && num[i] == num[i - 1]) { // skip duplicate
continue;
}
for (int j = i + 1; j < num.length - 2; ++j) {
if (j > i + 1 && num[j] == num[j - 1]) { // skip duplicate
continue;
}
int k = j + 1;
int l = num.length - 1;
while (k < l) {
int sum = num[i] + num[j] + num[k] + num[l];
if (sum < target) {
++k;
while (k > j + 1 && k < l && num[k] == num[k - 1]) { // skip duplicate
++k;
}
}
else if (sum > target) {
--l;
while (k < l && num[l] == num[l + 1]) { // skip duplicate
--l;
}
}
else {
ArrayList<Integer> sol = new ArrayList<Integer>();
sol.add(num[i]);
sol.add(num[j]);
sol.add(num[k]);
sol.add(num[l]);
res.add(sol);
++k;
--l;
while (k > j + 1 && k < l && num[k] == num[k - 1]) { // skip duplicate
++k;
}
while (k < l && num[l] == num[l + 1]) { // skip duplicate
--l;
}
}
}
}
}
return res;
}
}
}