Given a string containing only three types of characters: '(', ')' and '', write a function to check whether this string is valid. We define the validity of a string by these rules:
Any left parenthesis '(' must have a corresponding right parenthesis ')'.
Any right parenthesis ')' must have a corresponding left parenthesis '('.
Left parenthesis '(' must go before the corresponding right parenthesis ')'.
'' could be treated as a single right parenthesis ')' or a single left parenthesis '(' or an empty string.
An empty string is also valid.
Solution：count
思路： 类似只有'(' 和 ')' 的情况，count分别加减1，看是否最终等于0. 这里存在*情况，所以变为scope[low, high]
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Time Complexity: O(N) Space Complexity: O(1)
Solution Code：
class Solution {
public boolean checkValidString(String s) {
int low = 0;
int high = 0;
for (int i = 0; i < s.length(); i++) {
if (s.charAt(i) == '(') {
low++;
high++;
} else if (s.charAt(i) == ')') {
if (low > 0) {
low--;
}
high--;
} else {
if (low > 0) {
low--;
}
high++;
}
if (high < 0) {
return false;
}
}
return low == 0;
}
}
