44import java .util .Iterator ;
55import java .util .LinkedList ;
66import java .util .List ;
7+ import java .util .Stack ;
78
89/**
10+ * Date 04/17/2017
11+ * @author Tushar Roy
12+ *
913 * Given a nested list of integers, implement an iterator to flatten it.
1014 * Each element is either an integer, or a list -- whose elements may also be integers or other lists.
1115 * Example 1:
1519 * https://leetcode.com/problems/flatten-nested-list-iterator/
1620 */
1721public class NestedIterator implements Iterator <Integer > {
18- 19- private final Deque <NestedInteger > stack ;
20- private NestedInteger current ;
21- private final Deque <Integer > positionStack ;
22- private int position = 0 ;
23- 22+ Stack <Iterator <NestedInteger >> stack = new Stack <>();
23+ Integer nextVal = null ;
2424 public NestedIterator (List <NestedInteger > nestedList ) {
25- NestedInteger ni = new NestedIntegerImpl (nestedList );
26- current = ni ;
27- this .stack = new LinkedList <>();
28- this .positionStack = new LinkedList <>();
25+ stack .push (nestedList .iterator ());
2926 }
3027
3128 @ Override
3229 public Integer next () {
33- return current .getList ().get (position ++).getInteger ();
30+ if (!hasNext ()) {
31+ throw new IllegalArgumentException ();
32+ }
33+ Integer val = nextVal ;
34+ nextVal = null ;
35+ return val ;
3436 }
3537
3638 @ Override
3739 public boolean hasNext () {
38- if (position < current .getList ().size ()) {
39- NestedInteger i = current .getList ().get (position );
40- if (i .isInteger ()) {
41- return true ;
42- } else {
43- stack .offerFirst (current );
44- positionStack .offerFirst (position + 1 );
45- current = i ;
46- position = 0 ;
47- return hasNext ();
40+ if (nextVal != null ) {
41+ return true ;
42+ }
43+ while (!stack .isEmpty ()) {
44+ boolean pushedIntoStack = false ;
45+ Iterator <NestedInteger > itr = stack .peek ();
46+ if (itr .hasNext ()) {
47+ NestedInteger ni = itr .next ();
48+ if (ni .isInteger ()) {
49+ nextVal = ni .getInteger ();
50+ return true ;
51+ } else {
52+ pushedIntoStack = true ;
53+ stack .push (ni .getList ().iterator ());
54+ }
4855 }
49- } else {
50- if (stack .isEmpty ()) {
51- return false ;
56+ if (!pushedIntoStack ) {
57+ stack .pop ();
5258 }
53- current = stack .pollFirst ();
54- position = positionStack .pollFirst ();
55- return hasNext ();
5659 }
57- }
58- }
59- 60- class NestedIntegerImpl extends NestedInteger {
61- 62- private List <NestedInteger > list ;
63- NestedIntegerImpl (List <NestedInteger > list ) {
64- this .list = list ;
65- }
66- 67- @ Override
68- public boolean isInteger () {
6960 return false ;
7061 }
71- 72- @ Override
73- public Integer getInteger () {
74- return null ;
75- }
76- 77- @ Override
78- public List <NestedInteger > getList () {
79- return list ;
80- }
8162}
8263
83- 84- abstract class NestedInteger {
85- abstract boolean isInteger ();
86- abstract Integer getInteger ();
87- abstract List <NestedInteger > getList ();
64+ interface NestedInteger {
65+ boolean isInteger ();
66+ Integer getInteger ();
67+ List <NestedInteger > getList ();
8868}
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