Add Dart solution for Sqrt(x)

dart/sqrtx.dart

@@ -0,0 +1,33 @@
+/*
+Given a non-negative integer x, return the square root of x rounded down to the nearest integer.
+
+The returned integer should be non-negative as well.
+*/
+
+class Solution {
+  int mySqrt(int x) {
+    if (x < 2) {
+      return x;
+    }
+
+    int left = 1;
+    int right = x ~/ 2;
+    int ans = 1;
+
+    while (left <= right) {
+      final int mid = left + ((right - left) ~/ 2);
+      final int square = mid * mid;
+
+      if (square == x) {
+        return mid;
+      } else if (square < x) {
+        ans = mid;
+        left = mid + 1;
+      } else {
+        right = mid - 1;
+      }
+    }
+
+    return ans;
+  }
+}