22.平面列表

描述

给定一个列表，该列表中的每个要素要么是个列表，要么是整数。将其变成一个只包含整数的简单列表。

如果给定的列表中的要素本身也是一个列表，那么它也可以包含列表。

您在真实的面试中是否遇到过这个题？   是

样例

给定 [1,2,[1,2]]，返回 [1,2,1,2]。

给定 [4,[3,[2,[1]]]]，返回 [4,3,2,1]。

挑战

请用非递归方法尝试解答这道题。

这道题的递归方法较为简单，可以使用for循环对于列表进行处理：

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer,
 *     // rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds,
 *     // if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds,
 *     // if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */
class Solution {
public:
    // @param nestedList a list of NestedInteger
    // @return a list of integer
     vector<int> flatten(vector<NestedInteger> &nestedList) {
        // Write your code here
        vector<int> result;
        for(NestedInteger it : nestedList) {
            if(it.isInteger()) {
                result.push_back(it.getInteger());
            } else {
                vector<NestedInteger> nlist = it.getList();
                vector<int> ilist = flatten(nlist);
                result.insert(result.end(), ilist.begin(), ilist.end());
            }
        }
        return result;
    }
};

注意for(:)循环是用了C++11中新增的范围迭代语法。

不使用递归：

不是用递归就创建一个队列，反复多次循环将类中的非int类型转换为int类型保存。

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer,
 *     // rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds,
 *     // if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds,
 *     // if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */
class Solution {
public:
    // @param nestedList a list of NestedInteger
    // @return a list of integer
    vector<int> flatten(vector<NestedInteger> &nestedList) {
        // Write your code here
        vector<int> result;
        vector<NestedInteger> nlist = nestedList;
        int flag = 1;
        while(flag) {
            flag = 0;
            vector<NestedInteger> temp;
            for(NestedInteger it : nlist) {
                if(it.isInteger()) {
                    temp.push_back(it);
                } else {
                    vector<NestedInteger> list = it.getList();
                    temp.insert(temp.end(), list.begin(), list.end());
                    flag = 1;
                }
            }
            nlist = temp;
        }
        for(NestedInteger it : nlist) {
            result.push_back(it.getInteger());
        }
        return result;
    }
};