目录 
题目
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:
Given the list [[1,1],2,[1,1]],
By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].
Given the list [1,[4,[6]]],
By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].
解决方案
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* // 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 List<NestedInteger> list = new ArrayList<>();
private Stack<Integer> num = new Stack<>();
public NestedIterator(List<NestedInteger> nestedList) {
this.list = nestedList;
getFlatternInteger(list);
}
private void getFlatternInteger(List<NestedInteger> nest){
for(int k = nest.size() - 1; k >= 0; k--){
if(nest.get(k).isInteger()){
num.push(nest.get(k).getInteger());
} else {
List<NestedInteger> nestList = nest.get(k).getList();
getFlatternInteger(nestList);
}
}
}
@Override
public Integer next() {
return num.pop();
}
@Override
public boolean hasNext() {
return !num.isEmpty();
}
}
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i = new NestedIterator(nestedList);
* while (i.hasNext()) v[f()] = i.next();
*/
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注意事项
- 接口NestedInteger无需实现，可以直接调用相应方法。
- 私有方法getFlatternInteger用于对输入的List nestedList做展开并存入一个堆栈。对于是数字的项，直接存入堆栈，对于是List类型的元素，递归调用该方法。
- next()和hasNext()方法对栈做弹出和判空即可。
