给定长度为N的数组arr [] ，其中包含范围为[1，N]的数组元素，则任务是找到总和相等的对的最大数量，因为数组中的任何元素只能是单个对中的一部分。
例子：
Input: arr[] = {1, 4, 1, 4}
Output: 2
Explanation: Pairs {{1, 4}, {1, 4}} have equal sum 5.
Input: arr[] = {1, 2, 4, 3, 3, 5, 6}
Output: 3
Explanation: Pairs {{1, 5}, {2, 4}, {3, 3}} have equal sum 6.
方法：
可以从数组中获得的一对对的总和不能小于数组最小元素的2倍，也不能大于最大元素的2倍，因此我们找到了每个数组可以获取的最大对数。这些末端之间的总和，并在这些末端之间输出最大值。
实现此方法的方法如下：
- 存储给定数组的所有元素的频率。
- 遍历肢体之间的每个总和，并计算我们可以从这些总和中获得的最大对数。
- 打印获得的所有此类对中的最大计数以得出相同的总和。
下面是上述方法的实现：
C++
// C++ Program to implement
// the above approach
#include
using namespace std;
// Function to find the maximum count
// of pairs having equal sum
int maxCount(vector& freq, int mini, int maxi)
{
// Size of the array
int n = freq.size() - 1;
int ans = 0;
// Iterate through evey sum of pairs
// possible from the given array
for (int sum = 2 * mini; sum <= 2 * maxi; ++sum) {
// Count of pairs with given sum
int possiblePair = 0;
for (int firElement = 1; firElement < (sum + 1) / 2;
firElement++) {
// Check for a possible pair
if (sum - firElement <= maxi) {
// Update count of possible pair
possiblePair += min(freq[firElement],
freq[sum - firElement]);
}
}
if (sum % 2 == 0) {
possiblePair += freq[sum / 2] / 2;
}
// Update the answer by taking the
// pair which is maximum
// for every possible sum
ans = max(ans, possiblePair);
}
// Return the max possible pair
return ans;
}
// Function to return the
// count of pairs
int countofPairs(vector& a)
{
// Size of the array
int n = a.size();
int mini = *min_element(a.begin(), a.end()),
maxi = *max_element(a.begin(), a.end());
// Stores the frequencies
vector freq(n + 1, 0);
// Count the frequecy
for (int i = 0; i < n; ++i)
freq[a[i]]++;
return maxCount(freq, mini, maxi);
}
// Driver Code
int main()
{
vector a = { 1, 2, 4, 3, 3, 5, 6 };
// Function Call
cout << countofPairs(a) << endl;
} Java
// Java program to implement
// the above approach
class GFG{
// Function to find the maximum count
// of pairs having equal sum
static int maxCount(int[] freq,int maxi,int mini)
{
// Size of the array
int n = freq.length - 1;
int ans = 0;
// Iterate through evey sum of pairs
// possible from the given array
for(int sum = 2*mini; sum <= 2 * maxi; ++sum)
{
// Count of pairs with given sum
int possiblePair = 0;
for(int firElement = 1;
firElement < (sum + 1) / 2;
firElement++)
{
// Check for a possible pair
if (sum - firElement <= maxi)
{
// Update count of possible pair
possiblePair += Math.min(freq[firElement],
freq[sum - firElement]);
}
}
if (sum % 2 == 0)
{
possiblePair += freq[sum / 2] / 2;
}
// Update the answer by taking the
// pair which is maximum
// for every possible sum
ans = Math.max(ans, possiblePair);
}
// Return the max possible pair
return ans;
}
// Function to return the
// count of pairs
static int countofPairs(int[] a)
{
// Size of the array
int n = a.length;
// Stores the frequencies
int []freq = new int[n + 1];
int maxi = -1;
int mini = n+1;
for(int i = 0;iPython3
# Python3 program to implement
# the above approach
# Function to find the maximum count
# of pairs having equal sum
def maxCount(freq, maxi, mini):
# Size of the array
n = len(freq) - 1
ans = 0
# Iterate through evey sum of pairs
# possible from the given array
sum = 2*mini
while sum <= 2 * maxi:
# Count of pairs with given sum
possiblePair = 0
for firElement in range(1, (sum + 1) // 2):
# Check for a possible pair
if (sum - firElement <= maxi):
# Update count of possible pair
possiblePair += min(freq[firElement],
freq[sum - firElement])
sum += 1
if (sum % 2 == 0):
possiblePair += freq[sum // 2] // 2
# Update the answer by taking the
# pair which is maximum
# for every possible sum
ans = max(ans, possiblePair)
# Return the max possible pair
return ans
# Function to return the
# count of pairs
def countofPairs(a):
# Size of the array
n = len(a)
# Stores the frequencies
freq = [0] * (n + 1)
maxi = -1
mini = n+1
for i in range(len(a)):
maxi = max(maxi, a[i])
mini = min(mini, a[i])
# Count the frequecy
for i in range(n):
freq[a[i]] += 1
return maxCount(freq, maxi, mini)
# Driver Code
if __name__ == "__main__":
a = [1, 2, 4, 3, 3, 5, 6]
print(countofPairs(a))
# This code is contributed by chitranayalC#
// C# program to implement
// the above approach
using System;
class GFG {
// Function to find the maximum count
// of pairs having equal sum
static int maxCount(int[] freq)
{
// Size of the array
int n = freq.Length - 1;
int ans = 0;
// Iterate through evey sum of pairs
// possible from the given array
for (int sum = 2; sum <= 2 * n; ++sum) {
// Count of pairs with given sum
int possiblePair = 0;
for (int firElement = 1;
firElement < (sum + 1) / 2; firElement++) {
// Check for a possible pair
if (sum - firElement <= n) {
// Update count of possible pair
possiblePair
+= Math.Min(freq[firElement],
freq[sum - firElement]);
}
}
if (sum % 2 == 0) {
possiblePair += freq[sum / 2] / 2;
}
// Update the answer by taking the
// pair which is maximum
// for every possible sum
ans = Math.Max(ans, possiblePair);
}
// Return the max possible pair
return ans;
}
// Function to return the
// count of pairs
static int countofPairs(int[] a)
{
// Size of the array
int n = a.Length;
// Stores the frequencies
int[] freq = new int[n + 1];
// Count the frequecy
for (int i = 0; i < n; ++i)
freq[a[i]]++;
return maxCount(freq);
}
// Driver Code
public static void Main(String[] args)
{
int[] a = { 1, 2, 4, 3, 3, 5, 6 };
Console.Write(countofPairs(a) + "\n");
}
}
// This code is contributed by Amit Katiyar输出
3
时间复杂度： O(N 2 )
辅助空间： O(N)