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].

思路

【又是一道一开始没想出来的题】

数组套数组的情况非常复杂，但是我们很容易想到用深度遍历的方法来做。

深度遍历两种方法，一种递归，一种用栈。在这道题中，由于我们是要设计一个类，利用递归的方法就比较麻烦了。我们在类中维护一个栈，可以实现题意所提到的所有功能。

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/**
 * // 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 {
private:
    stack< NestedInteger > lists;
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        int size = nestedList.size();
        for (int i = (size - 1); i >= 0; i--){
            lists.push(nestedList[i]);
        }
    }
    int next() {
        NestedInteger cur = lists.top();
        lists.pop();
        return cur.getInteger();
    }
    bool hasNext() {
        //Use while here instead of if, to keep finding values after '[]'
        while(!lists.empty()){
            NestedInteger cur = lists.top();
            if (!cur.isInteger()){
                lists.pop();
                vector<NestedInteger> tmplist = cur.getList();
                int len = tmplist.size();
                for (int i = (len - 1); i >= 0; i--){
                    //cout << "Inside";
                    lists.push(tmplist[i]);
                }
            }
            else{
                return true;
            }
        }
        return false;
    }
};
/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */

参考资料： https://discuss.leetcode.com/topic/41846/concise-c-without-storing-all-values-at-initialization