3Sum
问题描述
Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.
Note: The solution set must not contain duplicate triplets.
For example, given array S = [-1, 0, 1, 2, -1, -4],
A solution set is:
[
[-1, 0, 1],
[-1, -1, 2]
]
分析
先对数组进行排序,然后固定左边值a,在它的右边夹逼求解是否存在中间值b和右边值c,满足a+b+c=0。
解答
class Solution {
public:
vector<vector<int>> threeSum(vector<int>& nums) {
vector<vector<int>> result;
int len = nums.size();
if (len < 3)
return result;
sort(nums.begin(), nums.end());
if (nums[0] > 0 || nums[len - 1] < 0)
return result;
for (int i = 0; ; i++)
{
int front = -nums[i];
int mid = i + 1;
int back = len - 1;
for ( ;mid < back; )
{
if (nums[back] < 0)
break;
int sum = nums[mid] + nums[back];
if (sum < front)
mid++;
else if (sum > front)
back--;
else
{
vector<int> findone(3, 0);
findone[0] = nums[i];
findone[1] = nums[mid++];
findone[2] = nums[back--];
result.push_back(findone);
while (nums[mid] == findone[1] && mid < back)
mid++;
while (nums[back] == findone[2] && mid < back)
back--;
}
}
while (i + 1 < len && nums[i] == nums[i + 1])
i++;
if (nums[i] > -1)
break;
}
return result;
}
};
3Sum Closest
问题描述
Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.
For example, given array S = {-1 2 1 -4}, and target = 1.
The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).
分析
和上一道题相类似的解法。
解答
class Solution {
public:
int threeSumClosest(vector<int>& nums, int target) {
int len = nums.size();
sort(nums.begin(), nums.end());
int result = nums[0] + nums[1] + nums[2];
for (int front = 0; front < len - 2; front++)
{
int mid = front + 1;
int back = len - 1;
int sum = 0;
for ( ;mid < back; )
{
sum = nums[front] + nums[mid] + nums[back];
if (sum == target)
return sum;
else if (sum < target)
mid++;
else
back--;
if (abs(target - sum) < abs(target - result))
result = sum;
}
}
return result;
}
};