246. Strobogrammatic Number

 

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.

Example 1:

Input:  "69"
Output: true

Example 2:

Input:  "88"
Output: true

Example 3:

Input:  "962"
Output: false

 

这道题定义了一种对称数,就是说一个数字旋转 180 度和原来一样,也就是倒过来看一样,比如 609,倒过来还是 609 等等,满足这种条件的数字其实没有几个,只有 0,1,8,6,9。这道题其实可以看做求回文数的一种特殊情况,还是用双指针来检测,首尾两个数字如果相等的话,只有它们是 0,1,8 中间的一个才行,如果它们不相等的话,必须一个是6一个是9,或者一个是9一个是6,其他所有情况均返回 false,参见代码如下;

 

解法一:

class Solution {
public:
    bool isStrobogrammatic(string num) {
        int l = 0, r = num.size() - 1;
        while (l <= r) {
            if (num[l] == num[r]) {
                if (num[l] != '1' && num[l] != '0' && num[l] != '8'){
                    return false;
                }
            } else {
                if ((num[l] != '6' || num[r] != '9') && (num[l] != '9' || num[r] != '6')) {
                    return false;
                }
            }
            ++l; --r;
        }
        return true;
    }
};

 

由于满足题意的数字不多,所以可以用 HashMap 来做,把所有符合题意的映射都存入哈希表中,然后双指针扫描,看对应位置的两个数字是否在哈希表里存在映射,若不存在,返回 false,遍历完成返回 true,参见代码如下:

 

解法二:

class Solution {
public:
    bool isStrobogrammatic(string num) {
        unordered_map<char, char> m {{'0', '0'}, {'1', '1'}, {'8', '8'}, {'6', '9'}, {'9', '6'}};
        for (int i = 0; i <= num.size() / 2; ++i) {
            if (m[num[i]] != num[num.size() - i - 1]) return false;
        }
        return true;
    }
};

 

Github 同步地址:

https://github.com/grandyang/leetcode/issues/246

 

类似题目:

Strobogrammatic Number II 

Strobogrammatic Number III 

 

参考资料:

https://leetcode.com/problems/strobogrammatic-number/

https://leetcode.com/problems/strobogrammatic-number/discuss/67182/Accepted-Java-solution

https://leetcode.com/problems/strobogrammatic-number/discuss/67257/5-lines-concise-and-easy-understand-C%2B%2B-solution

 

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