added solution & modified README.md for problem 121

leetcode/src/121.c

@@ -0,0 +1,27 @@
+int maxProfit(int* prices, int pricesSize){
+	/* If there is only only one day, profit cannot be made
+	 */
+	if(pricesSize <= 1) {
+        return 0;
+    }
+    int min_element = prices[0];
+    int max_difference = prices[1] - min_element;
+    for(int i = 0; i < pricesSize; i++) {
+		/* whenever maximum profit can be made, we sell the stock.
+           so we change the maximum difference
+		 */
+        if(prices[i] - min_element > max_difference) {
+            max_difference = prices[i] - min_element;
+		}
+		/* if a cheaper stock is available, we make that the minimum element
+		 */
+        if(min_element > prices[i]) {
+            min_element = prices[i];
+		}
+    }
+	/* return 0 if max_difference is less than zero, incase there is no way of making profits
+	 */
+    return (max_difference < 0)? 0 : max_difference;
+}
+
+