added Buy & Sell stock 3

Author: piyush6348

Arrays/Buy And Sell Stocks 3.cpp

@@ -0,0 +1,29 @@
+int maxProfit(vector<int>& prices) {
+        int n = prices.size();
+        if(n <= 1)
+            return 0;
+        if(n == 2) {
+            return max((prices[1] - prices[0]), 0);
+        }
+        
+        vector<int> secondMax(n,0);
+        
+        int maxPrice = prices[n-1];
+        int maxProfit = 0;
+        for(int i=n-2;i>=0;i--) {
+            maxProfit = max(maxProfit, maxPrice - prices[i]);
+            secondMax[i] = maxProfit;
+            maxPrice = max(maxPrice, prices[i]);
+        }
+        
+        int minPrice = prices[0];
+        int ans = 0;
+        for(int i=1;i<n;i++) {
+            int currentProfit = prices[i] - minPrice;
+            minPrice = min(minPrice, prices[i]);
+            ans = max(ans,currentProfit);
+            if(i != n-1) 
+                ans = max(ans, currentProfit + secondMax[i+1]);
+        }
+        return ans;
+    }