232 Implement Queue using Stacks

Author: int32bit

README.md

@@ -128,6 +128,7 @@
 + [228 Summary Ranges](algorithms/SummaryRanges)
 + [230 Kth Smallest Element in a BST(BST、中序遍历)](algorithms/KthSmallestElementinaBST)
 + [231 Power of Two(位运算，二进制1的个数)](algorithms/PowerofTwo)
++ [232 Implement Queue using Stacks(栈模拟队列)](algorithms/ImplementQueueusingStacks)
 
 ## Database
 

algorithms/ImplementQueueusingStacks/README.md

@@ -0,0 +1,57 @@
+## Implement Queue using Stacks 
+
+Implement the following operations of a queue using stacks.
+
+* push(x) -- Push element x to the back of queue.
+* pop() -- Removes the element from in front of queue.
+* peek() -- Get the front element.
+* empty() -- Return whether the queue is empty.
+Notes:
+
+* You must use only standard operations of a stack -- which means only `push to top`, `peek/pop from top`, `size`, and `is empty` operations are valid.
+* Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
+* You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
+
+## Solution
+
+使用两个栈模拟队列
+
+首先定义channel(first, second)操作，表示把第一个栈逐一弹出，并压入第二个栈:
+
+```cpp
+void channel(stack<int> &src, stack<int> &dest) {
+	while (!src.empty()) {
+		dest.push(src.top());
+		src.pop();
+	}
+}
+
+```
+
+* push(), push时往第一个栈压入
+* empty(), 当且仅当两个栈都为空时，队列空
+* peek(), 若第二个栈为空，则channel(first, second), 返回second.top()
+* pop(), 若第二个栈为空，则channel(first, second), second.pop()
+
+```cpp
+bool empty(void) {
+		return first.empty() && second.empty();
+}
+void push(int x) {
+	first.push(x);
+}
+int peek(void) {
+	if (second.empty()) {
+		channel(first, second);
+	}
+	return second.top();
+}
+void pop(void) {
+	if (second.empty()) {
+		channel(first, second);
+	}
+	second.pop();
+}
+```
+
+

algorithms/ImplementQueueusingStacks/solve.cpp

@@ -0,0 +1,43 @@
+#include <stack>
+#include <iostream>
+using namespace std;
+class Queue {
+	private:
+		stack<int> first;
+		stack<int> second;
+	public:
+		bool empty(void) {
+			return first.empty() && second.empty();
+		}
+		void push(int x) {
+			first.push(x);
+		}
+		int peek(void) {
+			if (second.empty()) {
+				channel(first, second);
+			}
+			return second.top();
+		}
+		void pop(void) {
+			if (second.empty()) {
+				channel(first, second);
+			}
+			second.pop();
+		}
+	private:
+		void channel(stack<int> &src, stack<int> &dest) {
+			while (!src.empty()) {
+				dest.push(src.top());
+				src.pop();
+			}
+		}
+};
+int main(int argc, char **argv)
+{
+	Queue queue;
+	queue.push(1);
+	queue.push(2);
+	queue.pop();
+	cout << queue.peek() << endl;
+	return 0;
+}