155. Min Stack

README.md

@@ -41,6 +41,9 @@ continually updating 😃.
 * [717. 1-bit and 2-bit Characters](src/0717_1_bit_and_2_bit_characters/1bitand2bitc.go)
 * [747. Largest Number At Least Twice of Others](./src/0747_largest_number_at_least_twice_of_others/largest_number_at_least_twice_of_others.go)
 
+### Stack
+* [155. Min Stack](src/0155_min_stack/min_stack.go)
+
 ### String
 * [3. Longest Substring Without Repeating Characters](./src/0003_longest_substring_without_repeating_characters/longest_substring_without_repeating_characters.go)   *`sliding window;`*  *`hash table`*
 * [14. Longest Common Prefix](src/0014_longest_common_prefix/lcp.go)

src/0155_min_stack/min_stack.go

@@ -0,0 +1,72 @@
+/*
+155. Min Stack
+https://leetcode.com/problems/min-stack/
+
+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.
+*/
+// time: 2019-01-10
+
+package minstack
+
+// MinStack a stack that supports push, pop, top, and retrieving the minimum element in constant time.
+type MinStack struct {
+	data []int
+}
+
+// Constructor initialize your data structure here.
+func Constructor() MinStack {
+	return MinStack{}
+}
+
+// Push element x onto stack.
+func (s *MinStack) Push(x int) {
+	s.data = append(s.data, x)
+}
+
+// Pop Removes the element on top of the stack.
+func (s *MinStack) Pop() {
+	if !s.IsEmpty() {
+		s.data = s.data[:s.GetSize()-1]
+	}
+}
+
+// Top  Get the top element.
+func (s *MinStack) Top() int {
+	//if s.IsEmpty(){
+	//	return 0
+	//}
+	// stack must have element.
+	return s.data[s.GetSize()-1]
+}
+
+// GetMin retrieving the minimum element in constant time.
+// time complexity: O(n)
+func (s *MinStack) GetMin() int {
+	//if s.IsEmpty() {
+	//	return 0
+	//}
+	// stack must have element.
+	stackSize := s.GetSize()
+	ret := s.data[stackSize-1]
+
+	for i := stackSize - 2; i >= 0; i-- {
+		if s.data[i] < ret {
+			ret = s.data[i]
+		}
+	}
+	return ret
+}
+
+// GetSize get size of the stack.
+func (s MinStack) GetSize() int {
+	return len(s.data)
+}
+
+func (s MinStack) IsEmpty() bool {
+	return s.GetSize() == 0
+}

src/0155_min_stack/min_stack_test.go

@@ -0,0 +1,21 @@
+package minstack
+
+import "testing"
+
+func TestMinStack(t *testing.T) {
+	obj := Constructor()
+	obj.Push(-2)
+	obj.Push(0)
+	obj.Push(-3)
+	if res := obj.GetMin(); res != -3 {
+		t.Errorf("expected %d, got %d", -3, res)
+	}
+
+	obj.Pop()
+	if res := obj.Top(); res != 0 {
+		t.Errorf("expected %d, got %d", 0, res)
+	}
+	if res := obj.GetMin(); res != -2 {
+		t.Errorf("expected %d, got %d", -2, res)
+	}
+}

src/README.md

@@ -60,6 +60,7 @@
 |0144|[144. Binary Tree Preorder Traversal](0144_binary_tree_preorder_traversal/binary_tree_preorder_traversal.go)|Medium|*`binary tree`*|
 |0150|[150. Evaluate Reverse Polish Notation](0150_evaluate_reverse_polish_notation/evaluate_reverse_polish_notation.go)|Medium|*`stack`*|
 |0153|[153. Find Minimum in Rotated Sorted Array](0153_find_minimum_in_rotated_sorted_array/fmirsa.go)|Medium|*`binary search`*|
+|0155|[155. Min Stack](0155_min_stack/min_stack.go)|Easy|*`stack`*|
 |0165|[165. Compare Version Numbers](0165_compare_version_numbers/compare_version_numbers.go)|Medium|*`string`*|
 |0167|[Two Sum II - Input array is sorted](./0167_two_sum2/two_sum2.go)|Easy|*`对撞指针(双索引)`*|
 |0198|[House Robber](./0198_house_robber/house_robber.go)|Easy|*`memory search;`* *`dynamic programming`*|