[Data structure and algorithm] Determine whether the character is unique

Implement an algorithm to determine whether all characters of a string s are all different.

1. Subject requirements

• Example 1:

	输入: s = "leetcode"
	输出: false 

• Example two:

	输入: s = "abc"
	输出: true

• limit:

	0 <= len(s) <= 100

Two, sample algorithm

• Implementation example one (Swift):
  Traverse the string, remove the first letter each time, and then query whether there is this removed first letter in the remaining string;

	func isUnique(_ astr: String) -> Bool {
    
    
        var s = astr
        for subS in s {
    
    
            s.removeFirst()
            if s.contains(subS) {
    
    
                return false
            }
        }
        return true
    }

• Implementation example two (C ):
  The i-th character is compared with the characters i+1 and after, if the same is found, false is returned directly, and there is no false return at the end of the loop, it means that everything is different, and true is returned;

	bool isUnique(char* astr){
    
    
	    int cnt = 0, i, j;
	    cnt = strlen(astr);
	    for (i = 0; i < cnt - 1; i++)
	        for (j = i + 1; j < cnt; j++)
	            if(astr[i] == astr[j])
	                return false;
	    return true;
	}

• Implementation example three (C++):

	bool isUnique(string astr) {
    
    
        for (int i = 0; i < astr.length(); ++i) {
    
    
            for (int j = i+1; j < astr.length(); ++j) {
    
    
                if (astr[i] == astr[j])
                	return false;
            }
        }
        return true;
    }