feat: add Online Stock Span (#1142)

* add Online Stock Span

* free obj

* Update leetcode/src/901.c

Co-authored-by: David Leal <halfpacho@gmail.com>

* Rename README.md to DIRECTORY.md

* merge conflicts

* chore: apply suggestions from code review

* chore: apply suggestions from code review

* Update leetcode/src/901.c

Co-authored-by: Taj <tjgurwara99@users.noreply.github.com>

Co-authored-by: David Leal <halfpacho@gmail.com>
Co-authored-by: Taj <tjgurwara99@users.noreply.github.com>

leetcode/DIRECTORY.md

@@ -87,6 +87,7 @@
 | 771  | [Jewels and Stones](https://leetcode.com/problems/jewels-and-stones/)                                                           | [C](./src/771.c)  | Easy       |
 | 852  | [Peak Index in a Mountain Array](https://leetcode.com/problems/peak-index-in-a-mountain-array/)                                 | [C](./src/852.c)  | Easy       |
 | 876  | [Middle of the Linked List](https://leetcode.com/problems/middle-of-the-linked-list/)                                           | [C](./src/876.c)  | Easy       |
+| 901  | [Online Stock Span](https://leetcode.com/problems/online-stock-span/)                                                           | [C](./src/901.c)  | Medium     |
 | 905  | [Sort Array By Parity](https://leetcode.com/problems/sort-array-by-parity/)                                                     | [C](./src/905.c)  | Easy       |
 | 917  | [Reverse Only Letters](https://leetcode.com/problems/reverse-only-letters/)                                                     | [C](./src/917.c)  | Easy       |
 | 938  | [Range Sum of BST](https://leetcode.com/problems/range-sum-of-bst/)                                                             | [C](./src/938.c)  | Easy       |

leetcode/src/901.c

@@ -0,0 +1,68 @@
+// Use monotonic stack.
+// Keep the stack of monotonically increasing price and index.
+
+// Runtime: O(n)
+// Space: O(n)
+typedef struct stack{
+    int price;
+    int index;
+    struct stack* previous;
+} Stack;
+
+
+typedef struct {
+    int index;
+    Stack* stackPointer;
+    Stack* sentry;
+} StockSpanner;
+
+
+StockSpanner* stockSpannerCreate() {
+    Stack* sentry = (Stack *)malloc(sizeof(Stack));
+    StockSpanner* result = (StockSpanner *)malloc(sizeof(StockSpanner));
+    result->index = 0;
+    result->sentry = sentry;
+    result->stackPointer = sentry;
+    return result;
+}
+
+int stockSpannerNext(StockSpanner* obj, int price) {
+    while(obj->stackPointer != obj->sentry && obj->stackPointer->price <= price){
+        Stack* currStackPointer = obj->stackPointer;
+        obj->stackPointer = obj->stackPointer->previous;
+        free(currStackPointer);
+    }
+    
+    obj->index += 1;
+    int result = obj->index;
+    if (obj->stackPointer != obj->sentry){
+        result -= obj->stackPointer->index;
+    }
+    
+    Stack* newStackItem = (Stack *)malloc(sizeof(Stack));
+    newStackItem->index = obj->index;
+    newStackItem->price = price;
+    newStackItem->previous = obj->stackPointer;
+    obj->stackPointer = newStackItem;
+    
+    return result;
+}
+
+void stockSpannerFree(StockSpanner* obj) {
+    while(obj->stackPointer != obj->sentry){
+        Stack* currStackPointer = obj->stackPointer;
+        obj->stackPointer = obj->stackPointer->previous;
+        free(currStackPointer);
+    }
+
+    free(obj->sentry);
+    free(obj);
+}
+
+/**
+ * Your StockSpanner struct will be instantiated and called as such:
+ * StockSpanner* obj = stockSpannerCreate();
+ * int param_1 = stockSpannerNext(obj, price);
+ 
+ * stockSpannerFree(obj);
+ */