add solution of problem 392: is subsequence

Author: ppulse

IsSubsequence392/README.md

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+Given a string **s** and a string **t**, check if **s** is subsequence of **t**.
+
+You may assume that there is only lower case English letters in both **s** and **t**. **t** is potentially a very long (length ~= 500,000) string, and **s** is a short string (<=100).
+
+A subsequence of a string is a new string which is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (ie, `"ace"` is a subsequence of `"abcde"` while `"aec"` is not).
+
+####Example 1:
+**s** = `"abc"`, **t** = `"ahbgdc"`
+
+Return `true`.
+
+####Example 2:
+s = `"axc"`, t = `"ahbgdc"`
+
+Return `false`.
+
+####Follow up:
+If there are lots of incoming S, say S1, S2, ... , Sk where k >= 1B, and you want to check one by one to see if T has its subsequence. In this scenario, how would you change your code?

IsSubsequence392/Solution.java

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+public class Solution {
+    public boolean isSubsequence(String s, String t) {
+        if (s == null && t == null)
+            return true;
+        else if (s == null || t == null)
+            return false;
+        
+        int idxOfS = 0;
+        for (int idxOfT = 0; idxOfT < t.length() && idxOfS < s.length(); idxOfT++) {
+            if (s.charAt(idxOfS) == t.charAt(idxOfT))
+                idxOfS++;
+        }
+        
+        return idxOfS == s.length();
+    }
+}