[LeetCode] 18. 4Sum 四数之和
Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums 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.
Example:
Given array nums = [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] ]
给一个含有n个整数的数组S， 找出所有不重复的4个数和等于target的所有解。
解法1：Two pointers, 跟3Sum差不多，只是扩展到4个数字的和。还是按照3Sum的解法，先排序，只是在外面套一层循环，相当于求n次3Sum，用set()去重。3Sum的时间复杂度是O(n^2)，所以总时间复杂度O(n^3)。但这个时间复杂度高，用时太长。
解法2：Hash map, 以空间换时间，首先建立一个Hash map，key值为数组中每两个元素的和，对应的value为这两个元素的下标组成的元组num[p]+num[q]] : (p,q) ，pairs元组不一定是唯一的。如对于num=[1,2,3,2]来说，dict={3:[(0,1),(0,3)], 4:[(0,2),(1,3)], 5:[(1,2),(2,3)]}。这样就可以检查target-key这个值在不在dict的key值中，如果target-key在dict中并且下标符合要求，那么就找到了这样的一组解。用set()去重。
Java: 2 pointers
public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
Arrays.sort(num);
HashSet<ArrayList<Integer>> hashSet = new HashSet<ArrayList<Integer>>();
ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
for (int i = 0; i < num.length; i++) {
for (int j = i + 1; j < num.length; j++) {
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) {
l--;
} else if (sum < target) {
k++;
} else if (sum == target) {
ArrayList<Integer> temp = new ArrayList<Integer>();
temp.add(num[i]);
temp.add(num[j]);
temp.add(num[k]);
temp.add(num[l]);
if (!hashSet.contains(temp)) {
hashSet.add(temp);
result.add(temp);
}
k++;
l--;
}
}
}
}
return result;
}
Java:
public class Solution {
int len = 0;
public List<List<Integer>> fourSum(int[] nums, int target) {
len = nums.length;
Arrays.sort(nums);
return kSum(nums, target, 4, 0);
}
private ArrayList<List<Integer>> kSum(int[] nums, int target, int k, int index) {
ArrayList<List<Integer>> res = new ArrayList<List<Integer>>();
if(index >= len) {
return res;
}
if(k == 2) {
int i = index, j = len - 1;
while(i < j) {
//find a pair
if(target - nums[i] == nums[j]) {
List<Integer> temp = new ArrayList<>();
temp.add(nums[i]);
temp.add(target-nums[i]);
res.add(temp);
//skip duplication
while(i<j && nums[i]==nums[i+1]) i++;
while(i<j && nums[j-1]==nums[j]) j--;
i++;
j--;
//move left bound
} else if (target - nums[i] > nums[j]) {
i++;
//move right bound
} else {
j--;
}
}
} else{
for (int i = index; i < len - k + 1; i++) {
//use current number to reduce ksum into k-1sum
ArrayList<List<Integer>> temp = kSum(nums, target - nums[i], k-1, i+1);
if(temp != null){
//add previous results
for (List<Integer> t : temp) {
t.add(0, nums[i]);
}
res.addAll(temp);
}
while (i < len-1 && nums[i] == nums[i+1]) {
//skip duplicated numbers
i++;
}
}
}
return res;
}
}
Python: Two pointers, Time: O(n^3), Space: O(1)
# Two pointer solution. (1356ms)
class Solution(object):
def fourSum(self, nums, target):
"""
:type nums: List[int]
:type target: int
:rtype: List[List[int]]
"""
nums.sort()
res = []
for i in xrange(len(nums) - 3):
if i and nums[i] == nums[i - 1]:
continue
for j in xrange(i + 1, len(nums) - 2):
if j != i + 1 and nums[j] == nums[j - 1]:
continue
sum = target - nums[i] - nums[j]
left, right = j + 1, len(nums) - 1
while left < right:
if nums[left] + nums[right] == sum:
res.append([nums[i], nums[j], nums[left], nums[right]])
right -= 1
left += 1
while left < right and nums[left] == nums[left - 1]:
left += 1
while left < right and nums[right] == nums[right + 1]:
right -= 1
elif nums[left] + nums[right] > sum:
right -= 1
else:
left += 1
return res
Python: HashMap, Time: O(n^2 * p), Space: O(n^2 * p)
# Hash solution. (224ms)
class Solution2(object):
def fourSum(self, nums, target):
"""
:type nums: List[int]
:type target: int
:rtype: List[List[int]]
"""
nums, result, lookup = sorted(nums), [], collections.defaultdict(list)
for i in xrange(0, len(nums) - 1):
for j in xrange(i + 1, len(nums)):
is_duplicated = False
for [x, y] in lookup[nums[i] + nums[j]]:
if nums[x] == nums[i]:
is_duplicated = True
break
if not is_duplicated:
lookup[nums[i] + nums[j]].append([i, j])
ans = {}
for c in xrange(2, len(nums)):
for d in xrange(c+1, len(nums)):
if target - nums[c] - nums[d] in lookup:
for [a, b] in lookup[target - nums[c] - nums[d]]:
if b < c:
quad = [nums[a], nums[b], nums[c], nums[d]]
quad_hash = " ".join(str(quad))
if quad_hash not in ans:
ans[quad_hash] = True
result.append(quad)
return result
Python:
def fourSum(self, nums, target):
def findNsum(nums, target, N, result, results):
if len(nums) < N or N < 2 or target < nums[0]*N or target > nums[-1]*N: # early termination
return
if N == 2: # two pointers solve sorted 2-sum problem
l,r = 0,len(nums)-1
while l < r:
s = nums[l] + nums[r]
if s == target:
results.append(result + [nums[l], nums[r]])
l += 1
while l < r and nums[l] == nums[l-1]:
l += 1
elif s < target:
l += 1
else:
r -= 1
else: # recursively reduce N
for i in range(len(nums)-N+1):
if i == 0 or (i > 0 and nums[i-1] != nums[i]):
findNsum(nums[i+1:], target-nums[i], N-1, result+[nums[i]], results)
results = []
findNsum(sorted(nums), target, 4, [], results)
return results
C++: 2 pointers
class Solution {
public:
vector<vector<int> > fourSum(vector<int> &nums, int target) {
set<vector<int> > res;
sort(nums.begin(), nums.end());
for (int i = 0; i < int(nums.size() - 3); ++i) {
for (int j = i + 1; j < int(nums.size() - 2); ++j) {
int left = j + 1, right = nums.size() - 1;
while (left < right) {
int sum = nums[i] + nums[j] + nums[left] + nums[right];
if (sum == target) {
vector<int> out;
out.push_back(nums[i]);
out.push_back(nums[j]);
out.push_back(nums[left]);
out.push_back(nums[right]);
res.insert(out);
++left; --right;
} else if (sum < target) ++left;
else --right;
}
}
}
return vector<vector<int> > (res.begin(), res.end());
}
};
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