这道题解法还挺多。只使用一个栈的方案，还需要再思考思考。
TODO
Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
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push(x) — Push element x onto stack.
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pop() — Removes the element on top of the stack.
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top() — Get the top element.
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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.
link:{sourcedir}/_0155_MinStack.java[role=include]