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:
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].

Example 2:
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].

java

/** * // 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> {        Stack<NestedInteger> stack = null;    public NestedIterator(List<NestedInteger> nestedList) {        stack = new Stack<NestedInteger>();        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() {        while (!stack.isEmpty()) {            NestedInteger curr = stack.peek();            if (curr.isInteger()) {                return true;            }            stack.pop();            for(int i = curr.getList().size() - 1; i >= 0; i--) {                stack.push(curr.getList().get(i));            }        }        return false;    }}/** * Your NestedIterator object will be instantiated and called as such: * NestedIterator i = new NestedIterator(nestedList); * while (i.hasNext()) v[f()] = i.next(); */

python

# """# This is the interface that allows for creating nested lists.# You should not implement it, or speculate about its implementation# """#class NestedInteger(object):#    def isInteger(self):#        """#        @return {boolean} True if this NestedInteger holds a single integer,#        rather than a nested list.#        """##    def getInteger(self):#        """#        @return {int} the single integer that this NestedInteger holds,#        if it holds a single integer#        Return None if this NestedInteger holds a nested list#        """##    def getList(self):#        """#        @return {NestedInteger[]} the nested list that this NestedInteger holds,#        if it holds a nested list#        Return None if this NestedInteger holds a single integer#        """class NestedIterator(object):        def __init__(self, nestedList):        # Initialize your data structure here.        self.st = nestedList[::-1]            # @return {int} the next element in the iteration    def next(self):        # Write your code here        return self.st.pop().getInteger()            # @return {boolean} true if the iteration has more element or false    def hasNext(self):        # Write your code here        while len(self.st) != 0:            if self.st[-1].isInteger():                return True            curr = self.st.pop()            self.st = self.st + curr.getList()[::-1]        return False# Your NestedIterator object will be instantiated and called as such:# i, v = NestedIterator(nestedList), []# while i.hasNext(): v.append(i.next())