This approach utilizes two stacks. One stack will hold the main elements, while the other stack will keep track of the minimum elements. When you push a value, it's added to the main stack, and if it's less than or equal to the current minimum, it's also pushed onto the min stack. During a pop operation, if the value being popped is the current minimum, it's also popped from the min stack.
Time Complexity: O(1) for each operation.
Space Complexity: O(n), where n is the number of elements in the stack.
1#include <stack>
2#include <climits>
3class MinStack {
4 std::stack<int> s;
5 std::stack<int> minStack;
6public:
7 MinStack() {
8 }
9
10 void push(int val) {
11 s.push(val);
12 if (minStack.empty() || val <= minStack.top()) {
13 minStack.push(val);
14 }
15 }
16
17 void pop() {
18 if (s.top() == minStack.top()) {
19 minStack.pop();
20 }
21 s.pop();
22 }
23
24 int top() {
25 return s.top();
26 }
27
28 int getMin() {
29 return minStack.top();
30 }
31};
32The C++ implementation uses two standard library stacks. The second stack, minStack, only stores the minimum values so far. Any time a value is pushed that's smaller or equal to the top of minStack, it's pushed onto minStack as well. When removing a value, if it's the same as the top of minStack, it's also popped from minStack.
This approach reduces space usage by storing a pair (value, minimum so far) in a single stack. Each element maintains the current minimum alongside its actual value, providing quick minimum retrieval by just examining the top of the stack.
Time Complexity: O(1) for each operation.
Space Complexity: O(n), each element only holds a pair so space is efficient relative to the number of elements.
1import java.util.Stack;
2
3class MinStack {
4 private Stack<int[]> stack;
5
6 public MinStack() {
7 stack = new Stack<>();
8 }
9
10 public void push(int val) {
11 if (stack.isEmpty()) {
12 stack.push(new int[]{val, val});
13 } else {
14 int currentMin = Math.min(val, stack.peek()[1]);
15 stack.push(new int[]{val, currentMin});
16 }
17 }
18
19 public void pop() {
20 stack.pop();
21 }
22
23 public int top() {
24 return stack.peek()[0];
25 }
26
27 public int getMin() {
28 return stack.peek()[1];
29 }
30}
31In Java, storing each stack element as an array lets you maintain both the value and the minimum value up to that point. It allows efficient retrieval of the minimum by inspecting the pair stored at any position in the stack, primarily the top.