add 155

Author: andavid

README.md

@@ -83,6 +83,7 @@
 | #    | Title                                       |
 | :--: | :------------------------------------------ |
 | 384  | [Shuffle an Array][384]                     |
+| 155  | [Min Stack][155]                            |
 
 
 [leetcode]: https://leetcode.com/problemset/all/
@@ -127,3 +128,4 @@
 [053]: https://github.com/andavid/leetcode-java/blob/master/note/053/README.md
 [198]: https://github.com/andavid/leetcode-java/blob/master/note/198/README.md
 [384]: https://github.com/andavid/leetcode-java/blob/master/note/384/README.md
+[155]: https://github.com/andavid/leetcode-java/blob/master/note/155/README.md

note/155/README.md

@@ -0,0 +1,68 @@
+# [Min Stack][title]
+
+## Description
+
+Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
+
+* push(x) -- Push element x onto stack.
+* pop() -- Removes the element on top of the stack.
+* top() -- Get the top element.
+* getMin() -- Retrieve the minimum element in the stack.
+
+**Example:**
+
+```
+MinStack minStack = new MinStack();
+minStack.push(-2);
+minStack.push(0);
+minStack.push(-3);
+minStack.getMin();   --> Returns -3.
+minStack.pop();
+minStack.top();      --> Returns 0.
+minStack.getMin();   --> Returns -2.
+```
+
+## 思路
+
+最容易想到的思路是，使用两个 Stack，一个栈正常 push 和 pop 数据，另外一个辅助栈只压入当前栈的最小值。
+
+也可以只使用一个栈。需要记录当前最小值，每次压入一个数时，如果它比当前最小值大，直接压入栈；如果比当前最小值还小，那么先压入当前最小值，然后更新当前最小值，最后再压入新的最小值。出栈的时候，如果值等于当前最小值，那么需要再次出栈，并更新当前最小值。
+
+## [完整代码][src]
+
+```java
+class MinStack {
+    int min;
+    Stack<Integer> stack;
+
+    public MinStack() {
+      min = Integer.MAX_VALUE;
+      stack = new Stack<Integer>();
+    }
+
+    public void push(int x) {
+      if (x <= min) {
+        stack.push(min);
+        min = x;
+      }
+      stack.push(x);
+    }
+
+    public void pop() {
+      if (stack.pop() == min) {
+        min = stack.pop();
+      }
+    }
+
+    public int top() {
+      return stack.peek();
+    }
+
+    public int getMin() {
+      return min;
+    }
+}
+```
+
+[title]: https://leetcode.com/problems/min-stack
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_155/Solution.java

src/com/andavid/leetcode/_155/Solution.java

@@ -0,0 +1,47 @@
+import java.util.*;
+
+class Solution {
+
+  public static class MinStack {
+    int min;
+    Stack<Integer> stack;
+
+    public MinStack() {
+      min = Integer.MAX_VALUE;
+      stack = new Stack<Integer>();
+    }
+
+    public void push(int x) {
+      if (x <= min) {
+        stack.push(min);
+        min = x;
+      }
+      stack.push(x);
+    }
+
+    public void pop() {
+      if (stack.pop() == min) {
+        min = stack.pop();
+      }
+    }
+
+    public int top() {
+      return stack.peek();
+    }
+
+    public int getMin() {
+      return min;
+    }
+  }
+
+  public static void main(String[] args) {
+    MinStack minStack = new MinStack();
+    minStack.push(-2);
+    minStack.push(0);
+    minStack.push(-3);
+    System.out.println(minStack.getMin());
+    minStack.pop();
+    System.out.println(minStack.top());
+    System.out.println(minStack.getMin());
+  }
+}