Added Stock Span Problem

Java/Stack/Stockspan.java

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+/*
+The stock span problem is a financial problem where we have a
+series of n daily price quotes for a stock and we need to calculate
+the span of stocks price for all n days.
+The span Si of the stocks price on a given day i is defined as the
+maximum number of consecutive days just before the given day,
+for which the price of the stock on the given day is less than or
+equal to its price on the current day.
+For example, if an array of 7 days prices is given as {100, 80, 60,
+70, 60, 75, 85}, then the span values for corresponding 7 days
+are {1, 1, 1, 2, 1, 4, 6}.
+
+*/
+
+class Stockspan {
+    // Function to calculate the span of stock’s price for all n days.
+    public static int[] calculateSpan(int price[], int n) {
+        int S[] = new int[n];
+        Stack<Integer> st = new Stack<>();
+        st.push(0);
+
+        // span value of first day is always 1.
+        S[0] = 1;
+
+        for (int i = 1; i < n; i++) {
+            // we pop elements from the stack till price at top of stack
+            // is less than or equal to current price.
+            while (!st.isEmpty() && price[i] >= price[st.peek()]) {
+                st.pop();
+            }
+
+            // if stack becomes empty, then price[i] is greater than all
+            // elements on left of it in list so span is i+1.
+            // Else price[i] is greater than elements after value at top of stack.
+            S[i] = st.isEmpty() ? i + 1 : i - st.peek();
+
+            // pushing this element to stack.
+            st.push(i);
+        }
+        // returning the list.
+        return S;
+    }
+}