## Flatten Nested List Iterator

07/18/2016 Stack Design

## Question

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,]],

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

## Solution

Result: Accepted Time: 32 ms

Here should be some explanations.

/**
* // 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:
queue<int> q;
void inqueue(vector<NestedInteger> &nestedList)
{
for(int i = 0; i < nestedList.size(); i++)
{
if(nestedList[i].isInteger())
{
q.push(nestedList[i].getInteger());
}
else
{
inqueue(nestedList[i].getList());
}
}
}
public:
NestedIterator(vector<NestedInteger> &nestedList) {
inqueue(nestedList);
}

int next() {
int x = q.front();
q.pop();
return x;
}

bool hasNext() {
return !q.empty();
}
};

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


Complexity Analytics

• Time Complexity: $O(n)$
• Space Complexity: $O(n)$