Strobogrammatic Number -- Leetcode 246

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down). Write a function to determine if a number is strobogrammatic. The number is represented as a string.

For example, the numbers "69", "88", and "818" are all strobogrammatic.

只要知道什么是strobogrammatic数， 这道题并不难。翻转180度对称的数字就那么几个：0->0, 1->1, 6->9, 8->8, 9->6,然后从两边向中间逐个比较对应位置的数字是否有映射关系。如果这个数包含有奇数个数字，中间那个只能是 0、1、8，不能是6和9。非常straightforward的C++ 实现如下。

class Solution {public:    bool isStrobogrammatic(string nums){    int start = 0, end = nums.length() - 1;

    while (start != end)    {            if ((nums[start] == 1 && nums[end] == 1) ||            (nums[start] == 8 && nums[end] == 8) || 

            (nums[start] ==6 && nums[end] == 9)  || 

            (nums[start] == 9 && nums[end] == 6)  )            {            ++start;            --end;            } else {            return false;            }    }    if (start == end)    {            if ((nums[start] == '0') || (nums[start] == '1') || (nums[start] == '8'))                return true;            else                return false;     }    }};

比上面代码更简洁的方法是建立一个查询表， 代码如下。

class Solution {public:    bool isStrobogrammatic(string nums) {unordered_map<char, char> mapping;mapping['0'] = '0';mapping['1'] = '1';mapping['6'] = '9';mapping['8'] = '8';mapping['9'] = '6';int start = 0, end = nums.length() - 1;while (start <= end){    if ((mapping.find(nums[start]) == mapping.end()) ||

                (mapping.find(nums[end]) == mapping.end()) ||

                (mapping[nums[start]] != nums[end]))

return false;

            ++start;    --end;}return true;    }};