题目描述

Given a word, you need to judge whether the usage of capitals in it is right or not.

We define the usage of capitals in a word to be right when one of the following >cases holds:

1. All letters in this word are capitals, like “USA”.
2. All letters in this word are not capitals, like “leetcode”.
3. Only the first letter in this word is capital, like “Google”.

Otherwise, we define that this word doesn’t use capitals in a right way.

例子

例子 1

Input: “USA” Output: True

例子 2

Input: “FlaG” Output: False

解题思路

这道题目比较简单,合法单词一共就三类: 1. 全大写, 2. 首字母大写, 3. 全小写; 首先通过首字母判断是不是全小写;再根据第二个字母判断是不是全大写;最后根据属于哪种情况遍历一遍字符串发现不合法直接返回 false 就可以了,代码如下:

class Solution {
public:
    bool detectCapitalUse(string word) {
        if (word.length() <= 1) return true;

        bool is_all_capital = false;
        if (isupper(word[1])) {
            if (!isupper(word[0])) return false;
            else is_all_capital = true;
        }

        for (int i = 2; i < word.length(); i++) {
            if (is_all_capital ^ (bool)(std::isupper(word[i]))) {
                return false;
            }
        }
        return true;
    }

};

• 时间复杂度: O(n)
• 空间复杂度: O(1)