feat: solve best time to buy and sell stock

Author: GangBean

best-time-to-buy-and-sell-stock/GangBean.java

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+class Solution {
+    public int maxProfit(int[] prices) {
+        /**
+        1. understanding
+        - price[i]: i th day's stock price
+        - to maximize profit, choose a single day to buy, and future day to sell.
+        - return maximum profit
+        - [7, 1, 5, 3, 6, 4] -> [0, 0, 4, 4, 5, 5]
+        - [7, 6, 4, 3, 1] -> [0, 0, 0, 0, 0]
+        2. strategy
+        - profit = (sell price) - (buy price)
+        3. complexity
+        - time: O(N)
+        - space: O(1)
+        */
+        int minPrice = prices[0];
+        for (int i = 0; i <prices.length; i++) {
+            int tmp = prices[i];
+            if (i == 0) {
+                prices[i] = 0;
+            } else {
+                prices[i] = Math.max(prices[i-1], prices[i] - minPrice);
+            }
+            minPrice = Math.min(minPrice, tmp);
+        }
+
+        return prices[prices.length - 1];
+    }
+}
+