add code of 122

Author: Li Li

Algorithms Basics/top-interview-questions/122. Best Time to Buy and Sell Stock II.cs

@@ -0,0 +1,44 @@
+// find each nearby peak , valley
+public class Solution {
+    public int MaxProfit(int[] prices) {
+        if (prices == null || prices.Length < 2) {
+            return 0;
+        }
+        int i = 0;
+        int valley = prices[0];
+        int peak = prices[0];
+        int maxProfit = 0;
+        while (i < prices.Length - 1) {
+            // find valley
+            while (i < prices.Length - 1 && prices[i] >= prices[i + 1]){
+                i++;
+            }
+            valley = prices[i];
+            // find peak
+            while (i < prices.Length - 1 && prices[i] <= prices[i + 1]) {
+                i++;
+            }
+            peak = prices[i];
+            maxProfit += peak - valley;
+        }
+        return maxProfit;
+    }
+}
+
+// if shape a ->b->c->d keep increasing, a as valley, d is peak, 
+// d - a = d- c + c - b + b -a;
+// so we don't need find exact peak d, just use d-c, c -b, b -a. then Sum.
+public class Solution {
+    public int MaxProfit(int[] prices) {
+        if (prices == null || prices.Length < 2) {
+            return 0;
+        }
+        int maxProfit = 0;
+        for (int i = 1; i < prices.Length; i++) {
+            if (prices[i] > prices[i - 1]) {
+                maxProfit += prices[i] - prices[i - 1];
+            }
+        }
+        return maxProfit;
+    }
+}