341.flatten-nested-list-iterator.java

/*
 * @lc app=leetcode id=341 lang=java
 *
 * [341] Flatten Nested List Iterator
 *
 * https://leetcode.com/problems/flatten-nested-list-iterator/description/
 *
 * algorithms
 * Medium (48.07%)
 * Total Accepted:    118.5K
 * Total Submissions: 243.7K
 * Testcase Example:  '[[1,1],2,[1,1]]'
 *
 * 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].
 *
 *
 *
 *
 */
/**
 * // 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 Iterator<NestedInteger> iter;
     private List<NestedInteger> integerList;

     public NestedIterator(List<NestedInteger> nestedList) {
         this.integerList = new LinkedList<>();
         flattenNestedList(nestedList);
         this.iter = integerList.iterator();
     }

     @Override
     public Integer next() {
         return iter.next().getInteger();
     }

     @Override
     public boolean hasNext() {
         return iter.hasNext();
     }

     private void flattenNestedList(List<NestedInteger> nestedList) {
         for (NestedInteger i : nestedList) {
             if (!i.isInteger()) {
                 flattenNestedList(i.getList());
             } else {
                 integerList.add(i);
             }
         }
     }
 }

 /**
  * Your NestedIterator object will be instantiated and called as such:
  * NestedIterator i = new NestedIterator(nestedList);
  * while (i.hasNext()) v[f()] = i.next();
  */