题目描述：
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
就是输入两个字符串，判断ransom是否为magazine的子集。
考查的是哈希表，hash[字符，出现次数]先遍历magazine递增次数，再遍历ransom递减次数，如果：
hash[ransomNote[i]]没有包含返回false；
hash[ransomNote[i]]为负数，返回false。
实现代码：
public class Solution {
public bool CanConstruct(string ransomNote, string magazine) {
var hash = new Dictionary<char,int>();
for(var i = 0;i < magazine.Length; i++){
if(!hash.ContainsKey(magazine[i])){
hash.Add(magazine[i],1);
}
else{
hash[magazine[i]]++;
}
}
for(var i = 0;i < ransomNote.Length; i++){
if(!hash.ContainsKey(ransomNote[i])){
return false;
}
else{
hash[ransomNote[i]] --;
}
}
foreach(var k in hash.Keys){
if(hash[k] < 0){
return false;
}
}
return true;
}
}