Add solution to "69. Sqrt(x)" in C++

Easy/sqrtx/solution.cpp

@@ -0,0 +1,16 @@
+class Solution {
+public:
+    int mySqrt(int x) {
+        if (x == 0) {
+            return 0;
+        }
+
+        int times = 20;
+        double xi = x;
+        for (int i = 0; i < times; ++i) {
+            xi = (xi + x / xi) / 2;
+        }
+
+        return (int)xi;
+    }
+};