# 341. Flatten Nested List Iterator (Medium)

https://leetcode.com/problems/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:

```Input: [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Explanation: By calling next repeatedly until hasNext returns false,
the order of elements returned by next should be: `[1,1,2,1,1]`.```

Example 2:

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

## Solutions

``````class NestedIterator implements Iterator<Integer> {

// The major point of this solution is flatten the input List<NestedInteger> as
// List<Integer> at first. Then process the flattened list when requests send in.

int pointer = 0;

List<Integer> flatten = new ArrayList<>();

public NestedIterator(List<NestedInteger> nestedList) {
if (nestedList == null || nestedList.size() == 0) {
return;
}

for (int i = 0; i < nestedList.size(); i++) {
recurse(nestedList.get(i));
}
}

private void recurse(NestedInteger ni) {
if (ni == null) {
return;
}

// if ni is an integer
if (ni.isInteger()) {

return;
}

// if ni is a list
for (NestedInteger itg : ni.getList()) {
recurse(itg);
}
}

@Override
public Integer next() {
if (hasNext()) {
return flatten.get(pointer++);
}

return null;
}

@Override
public boolean hasNext() {
return pointer < flatten.size();
}
}
``````