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();elsereturn false;if(!stack.isEmpty() && stack.peek() == 'a')stack.pop();elsereturn false;break;}}return stack.isEmpty(); }
}程序运行结果，Accepted.