LeetCode383. 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~