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341. Flatten Nested List Iterator
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].思考题：这道题还可以使用栈来实现。尝试一下。
参考资料
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: .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
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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();
}
/**
* Runtime: 2 ms, faster than 99.89% of Java online submissions for Flatten Nested List Iterator.
*
* Memory Usage: 42.9 MB, less than 5.00% of Java online submissions for Flatten Nested List Iterator.
*/
public class NestedIterator implements Iterator<Integer> {
private Queue<Integer> queue;
public NestedIterator(List<NestedInteger> nestedList) {
queue = new LinkedList<>();
dfs(nestedList);
}
private void dfs(List<NestedInteger> nestedList) {
if (Objects.isNull(nestedList) || nestedList.isEmpty()) {
return;
}
for (NestedInteger nestedInteger : nestedList) {
if (nestedInteger.isInteger()) {
queue.offer(nestedInteger.getInteger());
} else {
dfs(nestedInteger.getList());
}
}
}
@Override
public Integer next() {
return queue.poll();
}
@Override
public boolean hasNext() {
return !queue.isEmpty();
}
}
private void test() {
List<NestedInteger> nestedList = new ArrayList<>();
NestedIterator iterator = new NestedIterator(nestedList);
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}