LeetCode No341. Flatten Nested List Iterator

1. 題目描述

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].

2. 思路

1. 在構造函數中，利用題目提供的API將嵌套列表遞歸展開到vector<int>對象中，注意使用下標，可以省去列表的複製開銷。
2. 迭代器中使用iter_記錄迭代器當前位置。

3. 算法

/**
 * // 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) {
        Flaten(nestedList, 0, ivec_);
        iter_ = ivec_.begin();
    }

    int next() {
        return *iter_++;
    }

    bool hasNext() {
        if (iter_ != ivec_.end())  return true;
        else return false;
    }
private:
    void Flaten(const vector<NestedInteger> &nested_list, int idx, vector<int> &out_vec) {
        if (idx == nested_list.size())  return;
        if (nested_list[idx].isInteger()) {
            out_vec.push_back(nested_list[idx].getInteger());
        } else {
            Flaten(nested_list[idx].getList(), 0, out_vec);
        }
        Flaten(nested_list, idx + 1, out_vec);
    }
    vector<int> ivec_;
    vector<int>::iterator iter_;
};

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

4. 參考資料