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].

解题思路：借助一个栈把所有的元素先倒序存进去，每出来一个看是否是整数，如果不是依然倒序存进去，否则直接把这个数返回即可。

/**
 * // 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 {
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        for(int i = nestedList.size() - 1; i >= 0; i--) {
            s.push(nestedList[i]);
        }
    }

    int next() {
        NestedInteger top = s.top();
        s.pop();
        return top.getInteger();
    }

    bool hasNext() {
       while(!s.empty()) {
           NestedInteger top = s.top();
           if(top.isInteger())return true;
           s.pop();
           for(int i = top.getList().size() - 1; i >= 0; i--) {
               s.push(top.getList()[i]);
           }
       }
        return false;
    }
private:
    stack<NestedInteger>s;
};

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */