LeetCode刷题笔录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.

这题只要求括号是合法的。用一个stack，遇到左括号就丢到stack里，遇到右括号就弹出stack顶上的括号，看看是不是一对。是的话就继续，不是的话返回false。扫描完一遍以后，stack如果不是空的，说明左括号多了，返回false。

public class Solution {
public boolean isValid(String s) {
//this stack stores all the left parentheses
Stack<Character> stack = new Stack<Character>();
for(int i = 0; i < s.length(); i++){
char current = s.charAt(i);
//if the next character is a right par
if(isLeft(current) == false){
if(stack.isEmpty()){
return false;
}
//check the top par on the stack if it formsa pair with the current par
char top = stack.pop();
if(isPair(top, current) == false){
return false;
}
}
//if the next character is a left par, push onto the stack
else{
stack.push(current);
}
}
if(stack.isEmpty() == false)
return false;
return true;

}

public boolean isLeft(char c){
if(c == '[' || c == '{' || c == '(')
return true;
else
return false;
}

public boolean isPair(char left, char right){
if(left == '(' && right == ')')
return true;
if(left == '[' && right == ']')
return true;
if(left == '{' && right == '}')
return true;
return false;
}
}