BestTimetoBuyandSellStock*

Change-Id: I37a3b8ddeecc253528579180fadfaa8c996c6944

Author: applewjg

BestTimetoBuyandSellStock.java

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+/*
+ Author:     King, [email protected]
+ Date:       Oct 07, 2014
+ Problem:    Best Time to Buy and Sell Stock
+ Difficulty: Easy
+ Source:     http://leetcode.com/onlinejudge#question_121
+ Notes:
+ Say you have an array for which the ith element is the price of a given stock on day i.
+ If you were only permitted to complete at most one transaction (ie, buy one and sell one share of the stock), 
+ design an algorithm to find the maximum profit.
+
+ Solution: For each element, calculate the max difference with the former elements.
+ */
+
+public class Solution {
+    public int maxProfit(int[] prices) {
+        int n = prices.length;
+        if (n <= 1) return 0;
+        int res = 0, minVal = prices[0];
+        for (int i = 1; i < n; ++i) {
+            res = Math.max(res, prices[i] - minVal);
+            minVal = Math.min(minVal, prices[i]);
+        }
+        return res;
+    }
+}

BestTimetoBuyandSellStockII.java

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+/*
+ Author:     King, [email protected]
+ Date:       Jan 02, 2015
+ Problem:    Best Time to Buy and Sell Stock II
+ Difficulty: Medium
+ Source:     https://oj.leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/
+ Notes:
+ Say you have an array for which the ith element is the price of a given stock on day i.
+ Design an algorithm to find the maximum profit. You may complete as many transactions as you like 
+ (ie, buy one and sell one share of the stock multiple times). 
+ However, you may not engage in multiple transactions at the same time 
+ (ie, you must sell the stock before you buy again).
+
+ Solution: 1. At the beginning of the ascending order: buy.
+              At the ending of the ascending order: sell.
+ */
+public class Solution {
+    public int maxProfit(int[] prices) {
+        int res = 0;
+        for (int i = 1; i < prices.length; ++i) {
+            res += Math.max(0, prices[i] - prices[i-1]);
+        }
+        return res;
+    }
+}

BestTimetoBuyandSellStockIII.java

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+/*
+ Author:     King, [email protected]
+ Date:       Jan 02, 2015
+ Problem:    Best Time to Buy and Sell Stock III
+ Difficulty: Medium
+ Source:     https://oj.leetcode.com/problems/best-time-to-buy-and-sell-stock-iii/
+ Notes:
+ Say you have an array for which the ith element is the price of a given stock on day i.
+ Design an algorithm to find the maximum profit. You may complete at most two transactions.
+ Note:
+ You may not engage in multiple transactions at the same time (ie, you must sell the stock before you buy again).
+
+ Solution: dp. max profit =  max { l2r[0...i] + r2l[i+1...N-1] }.
+                         0 <= i <= N-1
+ */
+
+public class Solution {
+    public int maxProfit(int[] prices) {
+        int n = prices.length;
+        if (n <= 1) return 0;
+        int[] l2r = new int[n];
+        int[] r2l = new int[n];
+        l2r[0] = 0; r2l[n-1] = 0;
+        int minVal = prices[0], maxVal = prices[n-1];
+        for (int i = 1; i < n; ++i) {
+            l2r[i] = Math.max(l2r[i-1], prices[i] - minVal);
+            minVal = Math.min(minVal, prices[i]);
+        }
+        for (int i = n - 2; i >= 0; --i) {
+            r2l[i] = Math.max(r2l[i+1], maxVal - prices[i]);
+            maxVal = Math.max(maxVal, prices[i]);
+        }
+        int res = l2r[n-1];
+        for (int i = 0; i < n - 1; ++i) {
+            res = Math.max(res, l2r[i] + r2l[i+1]);
+        }
+        return res;
+    }
+}