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'
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();
}
}