This is the solution about Leetcode - Flatten Nested List Iterator.

Description

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].

Solution

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/**
* // 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 NestedIterator implements Iterator<Integer> {
private List<Integer> list;
private int index;
public NestedIterator(List<NestedInteger> nestedList) {
list = new ArrayList<>();
helper(nestedList,list);
}
private void helper(List<NestedInteger> nestedList,List<Integer> list) {
for(NestedInteger element : nestedList) {
if(element.isInteger()) {
list.add(element.getInteger());
}else {
helper(element.getList(),list);
}
}
}
@Override
public Integer next() {
return list.get(index++);
}
@Override
public boolean hasNext() {
return index<list.size();
}
}
/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i = new NestedIterator(nestedList);
* while (i.hasNext()) v[f()] = i.next();
*/