swift: add «901. Online Stock Span»

Signed-off-by: Matej Focko <me@mfocko.xyz>

swift/online-stock-span.swift

@@ -0,0 +1,23 @@
+class StockSpanner {
+    private var st: [(Int, Int)] = []
+
+    init() {}
+
+    func next(_ price: Int) -> Int {
+        var span = 1
+        while !st.isEmpty && st.last!.0 <= price {
+            span += st.last!.1
+            st.removeLast()
+        }
+
+        st.append((price, span))
+
+        return span
+    }
+}
+
+/**
+ * Your StockSpanner object will be instantiated and called as such:
+ * let obj = StockSpanner()
+ * let ret_1: Int = obj.next(price)
+ */