Problem statement

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

• All letters in this word are capitals, like "USA".

• All letters in this word are not capitals, like "leetcode".

• Only the first letter in this word is capital, like "Google".

Given a string word, return true if the usage of capitals in it is right.

Example 1

Input: word = "USA"
Output: true

Example 2

Input: word = "FlaG"
Output: false

Constraints

• 1 <= word.length <= 100,

• word consists of lowercase and uppercase English letters.

Solution

Only when the first two characters of the word are uppercase, the rest must be the same. Otherwise, the rest is always lowercase.

Code

#include <string>
#include <iostream>
using namespace std;
bool isValidCase(const char& c, const bool isLower) {
    if (isLower) {
        return 'a' <= c && c <= 'z';
    }
    return 'A' <= c && c <= 'Z';
}
bool detectCapitalUse(string word) {
    if (word.length() == 1) {
        return true;
    }
    bool isLower = true;
    if (isValidCase(word[0], false) && isValidCase(word[1], false)) {
        isLower = false;
    }
    for (int i = 1; i < word.length(); i++) {
        if (!isValidCase(word[i], isLower)) {
            return false;
        }
    }
    return true;
}
int main() {
    cout << detectCapitalUse("USA") << endl;
    cout << detectCapitalUse("FlaG") << endl;
    cout << detectCapitalUse("leetcode") << endl;
    cout << detectCapitalUse("Google") << endl;
}

Output:
1
0
1
1

Complexity

• Runtime: O(N), where N = word.length.

• Extra space: O(1).

Reference

• https://leetcode.com/problems/detect-capital/

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