Flatten Nested List Iterator

Flatten Nested List Iterator stack

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].
``````

Flatten Nested List Iterator Solution

``````/**
* // This is the interface that allows for creating nested lists.
* // You should not implement it, or speculate about its implementation
* public interface NestedInteger {
*
*     // @return true if this NestedInteger holds a single integer, rather than a nested list.
*     public boolean isInteger();
*
*     // @return the single integer that this NestedInteger holds, if it holds a single integer
*     // Return null if this NestedInteger holds a nested list
*     public Integer getInteger();
*
*     // @return the nested list that this NestedInteger holds, if it holds a nested list
*     // Return null if this NestedInteger holds a single integer
*     public List<NestedInteger> getList();
* }
*/
public class FlattenNestedListIterator implements Iterator<Integer> {
Stack<NestedInteger> stack = new Stack<NestedInteger>();

public NestedIterator(List<NestedInteger> nestedList) {
for(int i = nestedList.size() - 1; i >= 0; i--) {
stack.push(nestedList.get(i));
}
}

@Override
public Integer next() {
return stack.pop().getInteger();
}

@Override
public boolean hasNext() {
while(!stack.isEmpty()) {
NestedInteger current = stack.peek();

if(current.isInteger()) {
return true;
}

stack.pop();

for(int i = current.getList().size() - 1;  i >= 0; i--) {
stack.push(current.getList().get(i));
}
}

return false;
}
}``````