825. Friends Of Appropriate Ages

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: 3
Explanation: Friend requests are made 110 -> 100, 120 -> 110, 120 -> 100.

class Solution {
    public int numFriendRequests(int[] ages) {
        int[] map = new int[121]; //每个年龄有多少人, 优化1 不用map用常数大小数组，这样时间和空间都是O（1）
        for (int age : ages) map[age]++;
        int count = 0;
        for (int i = 0; i <= 120; i++) {
            for (int j = 0; j <= 120; j++) {
                if (valid(i, j)) {
                    count += map[i] * (i == j ? map[j] - 1 : map[j]); //注意i,j相等不能skip，因为可能多个相等年龄
                }
            }
        }
        return count;
    }
    public boolean valid(int a, int b) {
        return !(b <= 0.5 * a + 7 || b > a || (b > 100 && a < 100)); //其实条件2和3重复了
    }
}