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3.3 Imagine a (literal) stack of plates. If the stack gets too high, it might topple. Therefore, in real life, we would likely start a new stack when the previous stack exceeds some threshold. Implement a data structure SetOf Stacks that mimics this. SetOf Stacks should be composed of several stacks and should create a new stack once the previous one exceeds capacity. SetOf Stacks. push() and SetOf Stacks. pop() should behave identically to a single stack (that is, pop() should return the same values as it would if there were just a single stack).
FOLLOW UP
Implement a function popAt(int index) which performs a pop operation on a specific sub-stack.
这道题让我们实现一个多个栈的数据结构，这个多个栈顾名思义是由许多子栈组成的，每个子栈都有相同的大小，在压入栈的过程中，如果当前子栈满了，则新建一个栈，压入新值。在移除栈顶元素时，若最上面的子栈移除后为空时，移除该子栈。还是返回栈顶元素和判断栈是否为空，这些基本的栈操作都不是很难，只要注意需要的时候新建和销毁子栈就行了。难点在于Follow Up，让我们来实现一个popAt函数，这个函数是要对于任意一个子函数来移除栈顶元素，找到并移除子栈的站定元素并不难，难点在于如果我们默认子栈必须都存满的话，那么如果中间的子栈栈顶被移除了，上面紧挨的子栈的栈底元素就要移到下面，成为下面的子栈的栈顶元素，然后以此类推，每个中间的子栈都要装满，这个实现起来就比较复杂，我们需要用递归来做，我们需要实现一个leftShift的辅助函数，这个函数可以移除子栈的栈顶元素或者栈底元素，具体实现参见代码如下：
class SetOfStacks { public: SetOfStacks(int capacity) : _capacity(capacity) {} void push(int x) { stack<int> last; if (!_stacks.empty() && _stacks.back().size() < _capacity) { last = _stacks.back(); _stacks.pop_back(); } last.push(x); _stacks.push_back(last); } void pop() { if (!_stacks.empty()) { stack<int> last = _stacks.back(); last.pop(); if (last.empty()) _stacks.pop_back(); } } int top() { if (!_stacks.empty()) { stack<int> last = _stacks.back(); return last.top(); } return 0; } bool empty() { return _stacks.empty(); } void popAt(int index) { leftShift(index, true); } int leftShift(int index, bool removeTop) { stack<int> *cur = &_stacks[index]; int removed_item; if (removeTop) { removed_item = cur->top(); cur->pop(); } else { stack<int> tmp; while (!cur->empty() && cur->size() != 1) { tmp.push(cur->top()); cur->pop(); } removed_item = cur->top(); cur->pop(); while (!tmp.empty()) { cur->push(tmp.top()); tmp.pop(); } } if (cur->empty()) _stacks.erase(_stacks.begin() + index); else if (_stacks.size() > index + 1) { int val = leftShift(index + 1, false); cur->push(val); } return removed_item; } private: vector<stack<int> > _stacks; int _capacity; };
[CareerCup] 3.3 Set of Stacks 多个栈
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原文地址：http://www.cnblogs.com/grandyang/p/4677072.html
