[LintCode] 58 4Sum 解题报告

Description
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.


Notice
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.



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)



思路
先排序，然后遍历数组，对每一个元素进行3Sum。


Code

public class Solution {
    /**
     * @param numbers : Give an array numbersbers of n integer
     * @param target : you need to find four elements that's sum of target
     * @return : Find all unique quadruplets in the array which gives the sum of
     *           zero.
     */
    public ArrayList<ArrayList<Integer>> fourSum(int[] numbers, int target) {
        /* your code */
        
        ArrayList<ArrayList<Integer>> results = new ArrayList<>();
        
        Arrays.sort(numbers);
        
        for (int i = 0; i < numbers.length - 3; i++) {
            if (i > 0 && numbers[i] == numbers[i - 1]) {
                continue;
            }
            
            for (int j = i + 1; j < numbers.length - 2; j++) {
                if (j > i + 1 && numbers[j] == numbers[j - 1]) {
                    continue;
                }
                
                int left = j + 1;
                int right = numbers.length - 1;
                while (left < right) {
                    int sum = numbers[i] + numbers[j] + numbers[left] + numbers[right];
                    if (sum > target) {
                        right--;
                    }
                    else if (sum < target) {
                        left++;
                    }
                    else {
                        ArrayList<Integer> list = new ArrayList<>();
                        list.add(numbers[i]);
                        list.add(numbers[j]);
                        list.add(numbers[left]);
                        list.add(numbers[right]);
                        results.add(list);
                        left++;
                        right--;
                        
                        while (left < right && numbers[left] == numbers[left - 1]) {
                            left++;
                        }
                        while (left < right && numbers[right] == numbers[right + 1]) {
                            right--;
                        }
                    }
                }
            }
        }
        
        return results;
    }
}