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FourSum.java
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128 lines (117 loc) · 2.93 KB
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package codingInterview;
import java.util.ArrayList;
import java.util.Arrays;
class Quadruplet {
int a;
int b;
int c;
int d;
Quadruplet(int a, int b, int c, int d) {
this.a = a;
this.b = b;
this.c = c;
this.d = d;
}
}
public class FourSum {
public static void main(String[] args) {
// TODO Auto-generated method stub
int[] nums = { -1, 0, 1, -1, 2, -2, 5, -2, -3, -5, 7 };
int target = 0;
// ArrayList<Quadruplet> result = NaiveFourSum(nums, target);
ArrayList<Quadruplet> result = BetterFourSum(nums, target);
System.out.println(result.size());
for (Quadruplet res : result) {
System.out.println("[ " + res.a + ", " + res.b + ", " + res.c
+ ", " + res.d + " ]");
}
System.out.println();
}
/**
* use two pointers : start and end, O(n^3)
* @param nums
* @param target
* @return
*/
private static ArrayList<Quadruplet> BetterFourSum(int[] nums, int target) {
// TODO Auto-generated method stub
ArrayList<Quadruplet> result = new ArrayList<Quadruplet>();
Arrays.sort(nums);
int len = nums.length;
if (nums == null || len < 4) {
return result;
}
for (int i = 0; i < len - 3; i++) {
if (i == 0 || nums[i] > nums[i - 1]) {
for (int j = i + 1; j < len - 2; j++) {
if (j == 1 || nums[j] > nums[j - 1]) {
int start = j + 1;
int end = len - 1;
int negate = -(nums[i] + nums[j]);
while (start < end) {
if (nums[start] + nums[end] > negate) {
end--;
} else if (nums[start] + nums[end] < negate) {
start++;
} else {
Quadruplet quad = new Quadruplet(nums[i],
nums[j], nums[start], nums[end]);
result.add(quad);
start++;
end--;
while (start < end
&& nums[start] == nums[start - 1]) {
start++;
}
while (start < end
&& nums[end] == nums[end + 1]) {
end--;
}
}
}
}
}
}
}
return result;
}
/**
* can not handle duplicates, O(n^4)
*
* @param nums
* @param target
* @return
*/
private static ArrayList<Quadruplet> NaiveFourSum(int[] nums, int target) {
// TODO Auto-generated method stub
ArrayList<Quadruplet> result = new ArrayList<Quadruplet>();
Arrays.sort(nums);
int len = nums.length;
if (nums == null || len < 4) {
return result;
}
for (int i = 0; i < len - 3; i++) {
if (nums[i] > target)
break;
for (int j = i + 1; j < len - 2; j++) {
if (nums[i] + nums[j] > target && nums[j] > 0)
break;
for (int k = j + 1; k < len - 1; k++) {
if (nums[i] + nums[j] + nums[k] > target && nums[k] > 0)
break;
for (int l = k + 1; l < len; l++) {
if (nums[i] + nums[j] + nums[k] + nums[l] > target
&& nums[l] > 0)
break;
if (nums[i] + nums[j] + nums[k] + nums[l] == target) {
Quadruplet quad = new Quadruplet(nums[i], nums[j],
nums[k], nums[l]);
result.add(quad);
}
}
}
}
}
return result;
}
}