有效括号 Valid Parentheses
题目
给定一个只包括 '('，')'，'{'，'}'，'['，']' 的字符串，判断字符串是否有效。
以下为详细说明：
Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
An input string is valid if:
Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order. Note that an empty string is also considered valid.
Example 1:
Input: "()"
Output: true
Example 2:
Input: "()[]{}"
Output: true
Example 3:
Input: "(]"
Output: false
Example 4:
Input: "([)]"
Output: false
Example 5:
Input: "{[]}"
Output: true
思路&题解
class Solution {
public boolean isValid(String s) {
if (s.isEmpty()) {
return true;
}
Stack<Character> stack = new Stack<>();
for (int i = 0; i < s.length(); i++) {
char c = s.charAt(i);
if (c == '(' || c == '{' || c == '[') {
stack.push(c);
} else {
if (stack.isEmpty()) {
return false;
}
char top = stack.pop();
if (c == ')' && top == '(') {
continue;
}
if (c == '}' && top == '{') {
continue;
}
if (c == ']' && top == '[') {
continue;
}
return false;
}
}
return stack.isEmpty();
}
}