Merge pull request #274 from boom-jumper/master

添加0232用栈实现队列 python3 版本 和 添加0150逆波兰表达式求值 python3 版本

problems/0150.逆波兰表达式求值.md

@@ -224,6 +224,22 @@ var evalRPN = function(tokens) {
 };
 ```
 
+python3
+
+```python
+def evalRPN(tokens) -> int:
+    stack = list()
+    for i in range(len(tokens)):
+        if tokens[i] not in ["+", "-", "*", "/"]:
+            stack.append(tokens[i])
+        else:
+            tmp1 = stack.pop()
+            tmp2 = stack.pop()
+            res = eval(tmp2+tokens[i]+tmp1)
+            stack.append(str(int(res)))
+    return stack[-1]
+```
+
 
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 * 作者微信：[程序员Carl](https://mp.weixin.qq.com/s/b66DFkOp8OOxdZC_xLZxfw)

problems/0232.用栈实现队列.md

@@ -282,6 +282,50 @@ class MyQueue {
 
 
 Python：
+```python
+# 使用两个栈实现先进先出的队列
+class MyQueue:
+    def __init__(self):
+        """
+        Initialize your data structure here.
+        """
+        self.stack1 = list()
+        self.stack2 = list()
+
+    def push(self, x: int) -> None:
+        """
+        Push element x to the back of queue.
+        """
+        # self.stack1用于接受元素
+        self.stack1.append(x)
+
+    def pop(self) -> int:
+        """
+        Removes the element from in front of queue and returns that element.
+        """
+        # self.stack2用于弹出元素，如果self.stack2为[],则将self.stack1中元素全部弹出给self.stack2
+        if self.stack2 == []:
+            while self.stack1:
+                tmp = self.stack1.pop()
+                self.stack2.append(tmp)
+        return self.stack2.pop()
+
+    def peek(self) -> int:
+        """
+        Get the front element.
+        """
+        if self.stack2 == []:
+            while self.stack1:
+                tmp = self.stack1.pop()
+                self.stack2.append(tmp)
+        return self.stack2[-1]
+
+    def empty(self) -> bool:
+        """
+        Returns whether the queue is empty.
+        """
+        return self.stack1 == [] and self.stack2 == []
+```
 
 
 Go：