Merge pull request #353 from z80160280/z80160280-patch-2

Update 0121.买卖股票的最佳时机.md

problems/0121.买卖股票的最佳时机.md

@@ -244,6 +244,47 @@ class Solution { // 动态规划解法
 
 Python：
 
+> 贪心法：
+```python
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        low = float("inf")
+        result = 0
+        for i in range(len(prices)):
+            low = min(low, prices[i]) #取最左最小价格
+            result = max(result, prices[i] - low) #直接取最大区间利润
+        return result
+```
+
+> 动态规划：版本一
+```python
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        length = len(prices)
+        if len == 0:
+            return 0
+        dp = [[0] * 2 for _ in range(length)]
+        dp[0][0] = -prices[0]
+        dp[0][1] = 0
+        for i in range(1, length):
+            dp[i][0] = max(dp[i-1][0], -prices[i])
+            dp[i][1] = max(dp[i-1][1], prices[i] + dp[i-1][0])
+        return dp[-1][1]
+```
+
+> 动态规划：版本二
+```python
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        length = len(prices)
+        dp = [[0] * 2 for _ in range(2)] #注意这里只开辟了一个2 * 2大小的二维数组
+        dp[0][0] = -prices[0]
+        dp[0][1] = 0
+        for i in range(1, length):
+            dp[i % 2][0] = max(dp[(i-1) % 2][0], -prices[i])
+            dp[i % 2][1] = max(dp[(i-1) % 2][1], prices[i] + dp[(i-1) % 2][0])
+        return dp[(length-1) % 2][1]
+```
 
 Go：
 ```Go