leetcode 18. 4Sum

题目

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: 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] ]

解析

package leetcode;

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

public class Leet_18 {

    public List<List<Integer>> fourSum(int[] nums, int target) {
        ArrayList<List<Integer>> res = new ArrayList<>();
        int len = nums.length;
        if (nums == null || len < 4)
            return res;

        Arrays.sort(nums);
        int max = nums[len - 1];
        if (4 * nums[0] > target || 4 * max < target)
            return res;

        int i, z;
        for (i = 0; i < len; i++) {
            z = nums[i];
            if (i > 0 && z == nums[i - 1])
                continue;
            if (z + 3 * max < target)   //z is too small
                continue;
            if (4 * z > target) // z is too large
                break;
            if (4 * z == target) {
                if (i + 3 < len && nums[i + 3] == z)
                    res.add(Arrays.asList(z, z, z, z));
                break;
            }

            threeSumForFourSum(nums, target - z, i + 1, len - 1, res, z);
        }
        return res;
    }

    public void threeSumForFourSum(int[] nums, int target, int low, int high, ArrayList<List<Integer>> fourSumList, int z1) {
        if (low + 1 >= high)
            return ;

        int max = nums[high];
        if (3 * max < target || 3 * nums[low] > target)
            return ;

        int i, z;
        for (i = low; i < high - 1; i++) {
            z = nums[i];
            if (i > low && z == nums[i - 1])
                continue;
            if (z + 2 * max < target)
                continue;
            if (3 * z > target)
                break;
            if (3 * z == target) {
                if (i + 1 < high && nums[i + 2] == z)
                    fourSumList.add(Arrays.asList(z1, z, z, z));
                break;
            }
            twoSumForFourSum(nums, target - z, i + 1, high, fourSumList, z1, z);
        }
    }

    public void twoSumForFourSum(int[] nums, int target, int low, int high, ArrayList<List<Integer>> fourSumList, int z1, int z2) {
        if (low >= high)
            return ;
        if (2 * nums[low] > target || 2 * nums[high] < target)
            return ;

        int i = low, j = high, sum, x;
        while (i < j) {
            sum = nums[i] + nums[j];
            if (sum == target) {
                fourSumList.add(Arrays.asList(z1, z2, nums[i], nums[j]));
                x = nums[i];
                while (++i < j && x == nums[i]);

                x = nums[j];
                while (i < --j && x == nums[j]);
            }
            if (sum < target)
                i++;
            if (sum > target)
                j--;
        }
        return ;
    }
}