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].

 

 

stack。

思路：因为有时候会碰到暂存的情况，所以要用stack。但因为想要前面的nested最后先被叫到，所以每次压栈要从后往前压。

初始化：从后往前压一次。

next：直接pop顶层的integer。（默认顶层元素现在都是integer，会在hasNext里处理好）

hasNext：先while循环栈顶不是平int的情况，把nested展开从后往前再压进去子元素。如果处理完还是非空的话才是真的有next()。

 

细节：

1.处理展开顶层的逻辑要放在hasNext里，而不是next里。这样才能保证只有真的有数字才会叫next()，避免报runtime错误。比如corner case[[]]，其实里面什么integer都没有你根本不应该叫到next方法，初始化的时候stack里还是有元素的，所以你要在hasNext里展一下。 

 

实现：

/** * // 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
      
        getList(); * } */public class NestedIterator implements Iterator
       
         {    private Stack
        
          stack;    public NestedIterator(List
         
           nestedList) {        this.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() {        // P1: 注意这部分顶层处理要放在hasNext里不可以next里。不然对[[]]这种corner case会在hasNext()的时候好好的，在next()的时候runtime error。        while (!stack.isEmpty() && !stack.peek().isInteger()) {            List
          
            list = stack.peek().getList();            stack.pop();            for (int i = list.size() - 1; i >= 0; i--) {                stack.push(list.get(i));            }        }              return !stack.isEmpty();    }}/** * Your NestedIterator object will be instantiated and called as such: * NestedIterator i = new NestedIterator(nestedList); * while (i.hasNext()) v[f()] = i.next(); */