題目如下：

We are given that the string "abc" is valid.

From any valid string V, we may split V into two pieces X and Y such that X + Y (X concatenated with Y) is equal to V. (X or Y may be empty.) Then, X + "abc" + Y is also valid.

If for example S = "abc", then examples of valid strings are: "abc", "aabcbc", "abcabc", "abcabcababcc"

. Examples of invalid strings are: "abccba", "ab", "cababc", "bac".

Return true if and only if the given string S is valid.

Example 1:

Input: "aabcbc"
Output: true
Explanation: 
We start with the valid string "abc".
Then we can insert another "abc" between "a" and "bc", resulting in "a" + "abc" + "bc" which is "aabcbc".
 

Example 2:

Input: "abcabcababcc"
Output: true
Explanation: 
"abcabcabc" is valid after consecutive insertings of "abc".
Then we can insert "abc" before the last letter, resulting in "abcabcab" + "abc" + "c" which is "abcabcababcc".

Example 3:

Input: "abccba"
Output: false

Example 4:

Input: "cababc"
Output: false

Note:

1. 1 <= S.length <= 20000
2. S[i] is ‘a‘, ‘b‘, or ‘c‘

解題思路：這個題目有點用巧的意思了，可以逆向思維。每次從input中刪除掉一個"abc"，如果最後input能變成空就是True，否則就是False。

代碼如下：

class Solution(object):
    def isValid(self, S):
        """
        :type S: str
        :rtype: bool
        """
        while len(S) > 0:
            inx =  S.find(‘abc‘)
            if inx == -1:
                return False
            S = S[:inx] + S[inx+3:]
        return True

【leetcode】1003. Check If Word Is Valid After Substitutions