Approach: Three cases

def detectCapitalUse(s):
    return s.isupper() or s.islower() or (s[0].isupper() and s[1:].islower())

Complexity: O(n) time, O(1) space.

Full Solution — all languages

class Solution:
    def detectCapitalUse(self, s):
        return s.isupper() or s.islower() or (s[0].isupper() and s[1:].islower())

import java.util.*;
class Solution {
    public boolean detectCapitalUse(String s) {
        return s.equals(s.toUpperCase()) || s.equals(s.toLowerCase()) || (Character.isUpperCase(s.charAt(0)) && s.substring(1).equals(s.substring(1).toLowerCase()));
    }
}

#include <bits/stdc++.h> using namespace std;
class Solution {
    public: bool detectCapitalUse(string s) {
        auto isUpper = [](const string& x) {
            for (char c : x) if (!isupper((unsigned char)c)) return false;
            return true;
        };
        auto isLower = [](const string& x) {
            for (char c : x) if (!islower((unsigned char)c)) return false;
            return true;
        };
        if (isUpper(s) || isLower(s)) return true;
        return isupper((unsigned char)s[0]) && isLower(s.substr(1));
    }
};

/**
 * @param {string} s
 * @return {boolean}
 */ var detectCapitalUse = function(s) {
    const allUp = s === s.toUpperCase();
    const allLow = s === s.toLowerCase();
    const title = s[0] === s[0].toUpperCase() && s.slice(1) === s.slice(1).toLowerCase();
    return allUp || allLow || title;
};

Detect Capital

Example 1

Example 2

Example 3

Constraints