LeetCode No341. Flatten Nested List Iterator

1. 题目描述

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

在构造函数中，利用题目提供的API将嵌套列表递归展开到

vector<int>

对象中，注意使用下标，可以省去列表的复制开销。

迭代器中使用iter_记录迭代器当前位置。

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. 参考资料