Check If Word Is Valid After Substitutions

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 Ymay 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'

题目理解：

定义合法字符串：第一个合法字符串是“abc”，在合法字符串中间的任意一个位置插入“abc”也是合法字符串。给定一个字符串，判断其是否是合法字符串。

解题思路：

使用栈从左往右存储字符串中的每一个字符。

如果当前字符是a，那么直接入栈。

如果字符是b，而且栈顶是a，那么将a出栈，ab入栈；如果栈顶不是a，那么一定不是合法字符串

如果当前字符是c，而且栈顶是ab，那么ab出栈；如果栈顶不是ab，那么一定不是合法字符串

class Solution {
    public boolean isValid(String S) {
        int len = S.length();
        Stack<Integer> st = new Stack<>();
        for(char ch : S.toCharArray()){
            if(ch == 'a')
                st.push(1);
            else if(ch == 'b'){
                if(st.isEmpty())
                    return false;
                if(st.peek() == 1){
                    st.pop();
                    st.push(12);
                }
            }
            else if(ch == 'c'){
                if(st.isEmpty())
                    return false;
                if(st.peek() != 12)
                    return false;
                st.pop();
            }
        }
        return st.isEmpty();
    }
}