LeetCode 20. Valid Parentheses（括号校验）

原题网址：https://leetcode.com/problems/valid-parentheses/

Given a string containing just the characters

'('

,

')'

,

'{'

,

'}'

,

'['

and

']'

,
determine if the input string is valid.

The brackets must close in the correct order,

"()"

and

"()[]{}"

are
all valid but

"(]"

and

"([)]"

are
not.
方法：使用栈进行配对。

public class Solution {
public boolean isValid(String s) {
Stack<Character> stack = new Stack<>();
for(int i=0; i<s.length(); i++) {
char ch = s.charAt(i);
if (ch == '(' || ch == '[' || ch == '{') stack.push(ch);
else {
if (stack.isEmpty()) return false;
char last = stack.pop();
if ((last == '(' && ch == ')') || (last == '[' && ch == ']') || (last == '{' && ch == '}')) continue;
return false;
}
}
return stack.isEmpty();
}
}

另一种实现：使用数组实现栈。

public class Solution {
public boolean isValid(String s) {
char[] sa = s.toCharArray();
int stack = 0;
for(int i=0; i<sa.length; i++) {
if (sa[i]=='(' || sa[i]=='[' || sa[i]=='{') sa[stack++] = sa[i];
else if (stack == 0) return false;
else if (sa[stack-1] != '(' && sa[i]==')') return false;
else if (sa[stack-1] != '[' && sa[i]==']') return false;
else if (sa[stack-1] != '{' && sa[i]=='}') return false;
else stack --;
}
return stack == 0;
}
}