https://leetcode.com/problems/is-subsequence/#/solutions

http://www.cnblogs.com/EdwardLiu/p/6116896.html

public boolean isSubsequence(String s, String t) {
        int j = 0, i = 0;
        while (j < t.length() && i < s.length()) {
            if  (s.charAt(i) == t.charAt(j)) {
                i++;
                j++;
            } else {
                j++;
            }
               
       }
       if (i == s.length()) {
          return true;
       } else {
          return false;
       }
   }  

Follow Up:

The best solution is to create a map for String t, key is char, value is the index of appearance in ascending order

public boolean isSubsequence(String s, String t) {
        List<Integer>[] idx = new List[256]; // Just for clarity  字典集合, 每一個字母的ASCII 碼當作鍵, 在t中的順序當作值
        for (int i = 0; i < t.length(); i++) { // 生成字典
 

            if (idx[t.charAt(i)] == null)
                idx[t.charAt(i)] = new ArrayList<>();
            idx[t.charAt(i)].add(i);
        }
        
        int prev = 0;  // 前一個字母在t中的坐標, 控制升序
        for (int i = 0; i < s.length(); i++) {  //開始在字典裏查找
            if (idx[s.charAt(i)] == null) return false; // Note: char of S does NOT exist in T causing NPE
            int j = Collections.binarySearch(idx[s.charAt(i)], prev);  //在當前字母表中查找其比之前的字母升序的坐標
            if (j < 0) j = -j - 1;                                     // 二分搜索的註意點
            if (j == idx[s.charAt(i)].size()) return false;            //在升序後找不到false
            prev = idx[s.charAt(i)].get(j) + 1;                        //在t中查找當前s的字母比s中的前一個字母在t中升序的坐標

　　　　} 
　　　　return true; 
}

392. Is Subsequence