20. Valid Parentheses（stack）

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.

public class Solution {

if (s == null)
return false;
Stack<Character> stack = new Stack<Character>();
char[] ch = s.toCharArray();
for (int i = 0; i < ch.length; i++) {
if ((ch[i] == '(') || (ch[i] == '[') || (ch[i] == '{'))
stack.push(ch[i]);
else {
if (stack.isEmpty())
return false;
if (ch[i] - stack.pop() > 2)
return false;
}
}
return stack.isEmpty();

　　

public boolean isValid(String s) {
if (s == null)                       //改进1：可以不用String.charat(i)

return false;          //改进2： 先判断长度
//改进三： 不需要这么多if else ，符号的判断可以用ascii码
Stack<Character> stack = new Stack<Character>();
char[] ch = s.toCharArray();
for (int i = 0; i < ch.length; i++) {
if ((ch[i] == '(') || (ch[i] == '[') || (ch[i] == '{'))
stack.push(ch[i]);
else {
if (ch[i] == ')') {
if (stack.isEmpty())
return false;
else if (stack.pop() != '(')
return false;
}
if (ch[i] == ']') {
if (stack.isEmpty())
return false;
if (stack.pop() != '[')
return false;
}
if (ch[i] == '}') {
if (stack.isEmpty())
return false;
if (stack.pop() != '{')
return false;
}
}
}
if (!stack.isEmpty())
return false;
else
return true;
}
}