Problem
Some people will make friend requests. The list of their ages is given and ages[i] is the age of the ith person.
Person A will NOT friend request person B (B != A) if any of the following conditions are true:
age[B] <= 0.5 * age[A] + 7
age[B] > age[A]
age[B] > 100 && age[A] < 100
Otherwise, A will friend request B.
Note that if A requests B, B does not necessarily request A. Also, people will not friend request themselves.
How many total friend requests are made?
Example 1:
Input: [16,16]
Output: 2
Explanation: 2 people friend request each other.
Example 2:
Input: [16,17,18]
Output: 2
Explanation: Friend requests are made 17 -> 16, 18 -> 17.
Example 3:
Input: [20,30,100,110,120]
Output:
Explanation: Friend requests are made 110 -> 100, 120 -> 110, 120 -> 100.
Notes:
1 <= ages.length <= 20000.
1 <= ages[i] <= 120.
Solution
class Solution {
public int numFriendRequests(int[] ages) {
//0.5*ages[A]+7 < ages[B] <= ages[A]
//7 < 0.5*ages[A] ====> ages[A] >= 15
int res = 0;
int[] nums = new int[121];
int[] sums = new int[121];
for (int age: ages) {
nums[age]++;
}
for (int i = 1; i <= 120; i++) {
sums[i] = sums[i-1]+nums[i];
}
for (int i = 15; i <= 120; i++) {
if (nums[i] == 0) continue;
int count = sums[i] - sums[i/2+7]; //number of range B
res += count*nums[i] - nums[i]; //range B * number of age A
}
return res;
}
}
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