小白笔记--------------------------leetcode(20. 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.

这道题用到了栈，遍历字符串，将所有左括号压入栈中，这里有个重要的点，那就是右括号总是与离他最近的左括号配对，

所以接下来找到右括号之后，直接查询栈顶，如果配对则推出，不配对则不做操作，最后判断栈是否为空，也就是全部配对

成功，来判断最终结果。

代码：

public class Solution {
public boolean isValid(String s) {
boolean result = true;
if(s.length() % 2 != 0){
return false;
}
Stack a = new Stack();
char[] b = s.toCharArray();
for(int i = 0; i < s.length();i++){//全部压入栈中
if(b[i] == '(' || b[i] == '{' || b[i] == '['){
a.push(b[i]);
}
if(b[i] == ')' || b[i] == '}' || b[i] == ']' ){
if(a.empty()){
result = false;
}else{
if(((char)a.peek() + 1 == b[i]) || ((char)a.peek() + 2 == b[i])){
a.pop();
}
}
}
}
if(a.empty()){
result = true;
}else{
result = false;
}

return result;
}
}