Add solution 155 [Java]

Author: yanglbme

README.md

@@ -33,6 +33,7 @@ Complete [solutions](https://github.com/doocs/leetcode/tree/master/solution) to
 | 083 | [Remove Duplicates from Sorted List](https://github.com/doocs/leetcode/tree/master/solution/083.Remove%20Duplicates%20from%20Sorted%20List) | `Linked List` |
 | 136 | [Single Number](https://github.com/doocs/leetcode/tree/master/solution/136.Single%20Number) | `Hash Table`, `Bit Manipulation` |
 | 141 | [Linked List Cycle](https://github.com/doocs/leetcode/tree/master/solution/141.Linked%20List%20Cycle) | `Linked List`, `Two Pointers` |
+| 155 | [Min Stack](https://github.com/doocs/leetcode/tree/master/solution/155.Min%20Stack) | `Stack`, `Design` |
 | 189 | [Rotate Array](https://github.com/doocs/leetcode/tree/master/solution/189.Rotate%20Array) | `Array` |
 | 198 | [House Robber](https://github.com/doocs/leetcode/tree/master/solution/198.House%20Robber) | `Dynamic Programming` |
 | 203 | [Remove Linked List Elements](https://github.com/doocs/leetcode/tree/master/solution/203.Remove%20Linked%20List%20Elements) | `Linked List` |

solution/155.Min Stack/README.md

@@ -0,0 +1,65 @@
+## 最小栈
+### 题目描述
+
+设计一个支持 push，pop，top 操作，并能在常数时间内检索到最小元素的栈。
+
+- push(x) -- 将元素 x 推入栈中。
+- pop() -- 删除栈顶的元素。
+- top() -- 获取栈顶元素。
+- getMin() -- 检索栈中的最小元素。
+
+**示例:**
+```
+MinStack minStack = new MinStack();
+minStack.push(-2);
+minStack.push(0);
+minStack.push(-3);
+minStack.getMin();   --> 返回 -3.
+minStack.pop();
+minStack.top();      --> 返回 0.
+minStack.getMin();   --> 返回 -2.
+```
+
+### 解法
+创建两个栈 `stack`, `help`，`help`作为辅助栈，每次压栈时，若 `help` 栈顶元素大于/等于要压入的元素 `x`，或者 `help` 栈为空，则压入 `x`，否则重复压入栈顶元素。取最小元素时，从 `help` 中获取栈顶元素即可。
+
+```java
+class MinStack {
+    
+    private Stack<Integer> stack;
+    private Stack<Integer> help;
+
+    /** initialize your data structure here. */
+    public MinStack() {
+        stack = new Stack<>();
+        help = new Stack<>();
+    }
+    
+    public void push(int x) {
+        stack.push(x);
+        help.push(help.isEmpty() || help.peek() >= x ? x : help.peek());
+    }
+    
+    public void pop() {
+        stack.pop();
+        help.pop();
+    }
+    
+    public int top() {
+        return stack.peek();
+    }
+    
+    public int getMin() {
+        return help.peek();
+    }
+}
+
+/**
+ * Your MinStack object will be instantiated and called as such:
+ * MinStack obj = new MinStack();
+ * obj.push(x);
+ * obj.pop();
+ * int param_3 = obj.top();
+ * int param_4 = obj.getMin();
+ */
+```

solution/155.Min Stack/Solution.java

@@ -0,0 +1,38 @@
+class MinStack {
+    
+    private Stack<Integer> stack;
+    private Stack<Integer> help;
+
+    /** initialize your data structure here. */
+    public MinStack() {
+        stack = new Stack<>();
+        help = new Stack<>();
+    }
+    
+    public void push(int x) {
+        stack.push(x);
+        help.push(help.isEmpty() || help.peek() >= x ? x : help.peek());
+    }
+    
+    public void pop() {
+        stack.pop();
+        help.pop();
+    }
+    
+    public int top() {
+        return stack.peek();
+    }
+    
+    public int getMin() {
+        return help.peek();
+    }
+}
+
+/**
+ * Your MinStack object will be instantiated and called as such:
+ * MinStack obj = new MinStack();
+ * obj.push(x);
+ * obj.pop();
+ * int param_3 = obj.top();
+ * int param_4 = obj.getMin();
+ */