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].
题意：为一个 nestedList 实现一个 iterator。
解法一. 直接把 nestedlist 展开成一个 list. 代码
/**
* // 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> {
List<Integer> list = new ArrayList<>();
int index = 0;
public void helper(List<NestedInteger> nestedList, List<Integer> list, int index) {
if(index >= nestedList.size()) return;
NestedInteger cur = nestedList.get(index);
if(cur.isInteger()) {
list.add(cur.getInteger());
} else {
helper(cur.getList(), list, 0);
}
helper(nestedList, list, index + 1);
}
public NestedIterator(List<NestedInteger> nestedList) {
helper(nestedList, list, 0);
for(int i : list) {
System.out.println(i + " ");
}
}
@Override
public Integer next() {
return list.get(index++);
}
@Override
public boolean hasNext() {
return index < list.size();
}
}
解法二：使用 stack。在构造函数里，我们从后往前把值 push 到栈里。 当调用 hasNext func 的时候，如果 pop 出来是 list 的话，我们把 list 展开从后 push 到栈里。调用 next func， 我们直接返回栈顶。注意 hasNext func 里面的 while 是为了处理空 list 的情况。
public class NestedIterator implements Iterator<Integer> {
Stack<NestedInteger> stack;
public NestedIterator(List<NestedInteger> nestedList) {
stack = new Stack<>();
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() {
if(stack.isEmpty()) return false;
while(!stack.isEmpty()) {
NestedInteger cur = stack.pop();
if(cur.isInteger()) {
stack.push(cur);
return true;
} else {
for(int i = cur.getList().size() - 1; i >= 0; i--) {
stack.push(cur.getList().get(i));
}
}
}
return !stack.isEmpty();
}
}