lc341.java

package code;

import java.util.Iterator;
import java.util.List;
import java.util.Stack;

public class lc341 {
     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();
    }
    /**
     * Your NestedIterator object will be instantiated and called as such:
     * NestedIterator i = new NestedIterator(nestedList);
     * while (i.hasNext()) v[f()] = i.next();
     */
    public class NestedIterator implements Iterator<Integer> {
        Stack<NestedInteger> stack = new Stack<>();
        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 curr = stack.peek();
                if(curr.isInteger()) {
                    return true;
                }
                stack.pop();
                for(int i = curr.getList().size() - 1; i >= 0; i--) {
                    stack.push(curr.getList().get(i));
                }
            }
            return false;
        }
    }
}