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:
/**
* // 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<NestedInteger> st;
public NestedIterator(List<NestedInteger> nestedList) {
st = new Stack<>();
for(int i=nestedList.size()-1; i>=0; i--)
st.push(nestedList.get(i));
}
@Override
public Integer next() {
if(st.isEmpty()) return null;
return st.pop().getInteger();
}
@Override
public boolean hasNext() {
while(!st.isEmpty() && !st.peek().isInteger()){
NestedInteger ni = st.pop();
List<NestedInteger> list = ni.getList();
for(int i=list.size()-1; i>=0; i--)
st.push(list.get(i));
}
return !st.isEmpty();
}
}
/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i = new NestedIterator(nestedList);
* while (i.hasNext()) v[f()] = i.next();
*/