Leetcode 383 Ransom Note

Given
 an 
arbitrary
 ransom
 note
 string 
and 
another 
string 
containing 
letters from
 all 
the 
magazines,
 write 
a 
function 
that 
will 
return 
true 
if 
the 
ransom 
 note 
can 
be 
constructed 
from 
the 
magazines ; 
otherwise, 
it 
will 
return 
false. 



Each 
letter
 in
 the
 magazine 
string 
can
 only 
be
 used 
once
 in
 your 
ransom
 note.

Note:
You may assume that both strings contain only lowercase letters.

canConstruct("a", "b") -> false
canConstruct("aa", "ab") -> false
canConstruct("aa", "aab") -> true

题目大意：给两个string，判断第一个string能否由第二个string里面所含有的字母组成，
第二个string里面的所有字母只能使用一次

分析：建立一个hash数组，对第二个string遍历并记录每个字符出现的次数，然后遍历第一个string，
如果有出现hash里面不存在的字符，那么return false

class Solution {
public:
    bool canConstruct(string ransomNote, string magazine) {
        int a[26] = {0}, lenr = ransomNote.length(), lenm = magazine.length(), i;
        for(i = 0; i < lenm; i++)
            a[magazine[i] - 'a']++;
        for(i = 0; i < lenr; i++)
            if(a[ransomNote[i] - 'a'] == 0)
                return false;
            else
                a[ransomNote[i] - 'a']--;
        return true;
    }
};