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:
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].
Solution
TODO
Code
/**
* // 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> {
Stack<Iterator<NestedInteger>> s = new Stack<>();
Integer next = null;
public NestedIterator(List<NestedInteger> nestedList) {
s.push(nestedList.iterator());
computeNext();
}
@Override
public Integer next() {
int n = next;
next = null;
return n;
}
@Override
public boolean hasNext() {
computeNext();
return next != null;
}
private void computeNext() {
while(next == null && !s.isEmpty()) {
Iterator<NestedInteger> itr = s.peek();
// Allow empty list in the stack. We just clean that here.
if(!itr.hasNext()) {
s.pop();
continue;
}
NestedInteger n = itr.next();
if(n.isInteger()) next = n.getInteger();
else s.push(n.getList().iterator());
}
}
}
/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i = new NestedIterator(nestedList);
* while (i.hasNext()) v[f()] = i.next();
*/
Performance
TODO