Add solution

leetcode/0825.Friends-Of-Appropriate-Ages/825. Friends Of Appropriate Ages.go

@@ -1,6 +1,33 @@
 package leetcocde
 
+import "sort"
+
+// 解法一
 func numFriendRequests(ages []int) int {
+	return 0
+}
+
+// 解法二 双指针 + 排序 O(n logn)
+func numFriendRequests1(ages []int) int {
+	sort.Ints(ages)
+	left, right, res := 0, 0, 0
+	for _, age := range ages {
+		if age < 15 {
+			continue
+		}
+		for ages[left]*2 <= age+14 {
+			left++
+		}
+		for right+1 < len(ages) && ages[right+1] <= age {
+			right++
+		}
+		res += right - left
+	}
+	return res
+}
+
+// 解法三 暴力解法 O(n^2)
+func numFriendRequests2(ages []int) int {
 	res, count := 0, [125]int{}
 	for _, x := range ages {
 		count[x]++