add c# solution using two-pointer

Author: lazzerex

solution/1400-1499/1498.Number of Subsequences That Satisfy the Given Sum Condition/Solution.cs

@@ -0,0 +1,27 @@
+public class Solution {
+    public int NumSubseq(int[] nums, int target) {
+        const int MOD = 1000000007;
+
+        Array.Sort(nums);
+        int n = nums.Length;
+        int leftIndex = 0, rightIndex = n - 1;
+        int result = 0;
+
+        long[] pow2 = new long[n];
+        pow2[0] = 1;
+        for (int i = 1; i < n; i++) {
+            pow2[i] = (pow2[i - 1] * 2) % MOD;
+        }
+
+        while (leftIndex <= rightIndex) {
+            if (nums[leftIndex] + nums[rightIndex] <= target) {
+                result = (int)((result + pow2[rightIndex - leftIndex]) % MOD);
+                leftIndex++;
+            } else {
+                rightIndex--;
+            }
+        }
+
+        return result;
+    }
+}