[leetcode] 341. Flatten Nested List Iterator

Given a nested list of integers, implement an iterator to flatten it.

Each element is either an integer, or a list -- whose elements may also be integers or other lists.

Example 1:
Given the list [[1,1],2,[1,1]],

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2:
Given the list [1,[4,[6]]],

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

这道题是拉平层层嵌套的链表，题目难度为Medium。

这种嵌套的问题很自然想到要用栈来处理，由于要按照先后次序输出，所以要将链表从尾部开始依次进栈，然后判断栈顶元素是数字还是链表，如果是链表，将该链表出栈之后按同样的方法将它自身的元素从尾部开始进栈，直到栈顶元素是数字即可出栈输出了。这里将主要操作放在了hasNext()函数中，没有在next()函数中处理是为了避免空链表干扰hasNext()函数的判断。具体代码：

/** * // This is the interface that allows for creating nested lists. * // You should not implement it, or speculate about its implementation * class NestedInteger { *   public: *     // Return true if this NestedInteger holds a single integer, rather than a nested list. *     bool isInteger() const; * *     // Return the single integer that this NestedInteger holds, if it holds a single integer *     // The result is undefined if this NestedInteger holds a nested list *     int getInteger() const; * *     // Return the nested list that this NestedInteger holds, if it holds a nested list *     // The result is undefined if this NestedInteger holds a single integer *     const vector<NestedInteger> &getList() const; * }; */class NestedIterator {    stack<NestedInteger> stk;public:    NestedIterator(vector<NestedInteger> &nestedList) {        for(auto it=nestedList.rbegin(); it!=nestedList.rend(); ++it) {            stk.push(*it);        }    }    int next() {        auto nstInt = stk.top();        stk.pop();        return nstInt.getInteger();    }    bool hasNext() {        while(!stk.empty() && !stk.top().isInteger()) {            auto nstInt = stk.top();            stk.pop();            vector<NestedInteger> lst = nstInt.getList();            for(auto it=lst.rbegin(); it!=lst.rend(); ++it)                stk.push(*it);        }        if(stk.empty()) return false;        else return true;    }};/** * Your NestedIterator object will be instantiated and called as such: * NestedIterator i(nestedList); * while (i.hasNext()) cout << i.next(); */