feat(leetcode): add No.2824

Author: mickey0524

2824.Count-pairs-whose-sum-is-less-than-target.java

@@ -0,0 +1,32 @@
+// https://leetcode.com/problems/count-pairs-whose-sum-is-less-than-target/
+// algorithms
+// Easy (86.7%)
+// Total Accepted: 68.9K
+// Total Submissions: 79.5K
+
+
+class Solution {
+
+    public int countPairs(List<Integer> nums, int target) {
+        int len = nums.size();
+        if (len < 2) {
+            return 0;
+        }
+
+        if (len == 2) {
+            return nums.get(0) + nums.get(1) < target ? 1 : 0;
+        }
+
+        int res = 0;
+        for (int i = 0; i < len - 1; i++) {
+            for (int j = i + 1; j < len; j++) {
+                if (nums.get(i) + nums.get(j) < target) {
+                    res++;
+                }
+            }
+        }
+        
+        return res;
+    }
+
+}