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

判断字典里的字符能不能组成ransom note。

O(n)解法：

使用索引数组index存储每个字符在字典里找的值，使用方法indexOf(char ch, int offset)，更新index数组，如果某次查找出现-1，即找不到，这是就可判断false了

class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {
        if(ransomNote=="")
            return true;
        if(magazine=="")
            return false;
        int len=ransomNote.length();
        int[] index = new int[128];
        for(int i=0;i<len;i++){
            char cu=ransomNote.charAt(i);
            int result=magazine.indexOf(cu,index[cu]);
            if(result!=-1)
                index[cu]=result+1;
            else
                return false;
            
        }
        return true;
    }
}