p155

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LeetCode p155 Min Stack 题解

1.题目：

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.

题意：

写一个栈

2.解题思路:

见代码

3.代码

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public class MinStack {

	private long min;
	private Stack<Long> stack;

	/** initialize your data structure here. */
	public MinStack() {
		min = Integer.MAX_VALUE;
		stack = new Stack<Long>();
	}

	public void push(int x) {
		if (stack.size() == 0) {
			min = x;
			stack.push(0L);
		} else {
			stack.push(x - min);
			if (x < min)
				min = x;
		}
	}

	public void pop() {
		if (stack.isEmpty())
			return;
		Long i = stack.pop();
		if (i < 0) {
			min = min - i;
		}
	}

	public int top() {
		if (stack.peek() < 0) //开始这里写错了。。一直没过orz
			return (int) min;
		return (int) (stack.peek() + min);
	}

	public int getMin() {
		return (int) min;
	}
}

/**
 * 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();
 */

4.一些总结：