/** * // 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
     
       getList(); * } */public class NestedIterator implements Iterator
      
        {    Stack
       
         stack;    public NestedIterator(List
        
          nestedList) {        stack = new Stack<>();        pushData(nestedList);    }    @Override    public Integer next() {        return stack.pop().getInteger();    }    @Override    public boolean hasNext() {        while(!stack.isEmpty()) {            if (stack.peek().isInteger()) {                return true;            }            pushData(stack.pop().getList());        }        return false;    }        private void pushData(List
         
           nestedList) {        for (int i = nestedList.size() - 1; i >= 0; i--) {            stack.push(nestedList.get(i));        }    }}/** * Your NestedIterator object will be instantiated and called as such: * NestedIterator i = new NestedIterator(nestedList); * while (i.hasNext()) v[f()] = i.next(); */
         
        
       
      
     

 

1 Remember : Always make integer validation at hasNext() since it is too late to validate it at next().