LeetCode刷题：1003. Check If Word Is Valid After Substitutions
原题链接：https://leetcode.com/problems/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 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 <= S.length <= 20000 S[i] is 'a', 'b', or 'c'
算法设计
class Solution { public boolean isValid(String S) { Stack<Character> stack = new Stack<>(); for(char ch : S.toCharArray()){ switch(ch){ case 'a': case 'b': stack.push(ch); break; case 'c': if(!stack.isEmpty() && stack.peek() == 'b') stack.pop(); else return false; if(!stack.isEmpty() && stack.peek() == 'a') stack.pop(); else return false; break; } } return stack.isEmpty(); } }程序运行结果，Accepted.
