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.

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也是上古时期做的题了……其实现在要做就是一个stack的事，看到左括号就入栈，右括号就出栈，看是不是相对应的就好。以前的这种方法可以弃了。

public class Solution {
<span style="white-space:pre">	</span>public boolean isValid(String s) {

<span style="white-space:pre">		</span>String str = "";
<span style="white-space:pre">		</span>int len = s.length();
<span style="white-space:pre">		</span>for (int i = 0; i < len; i++) {
<span style="white-space:pre">			</span>// if(str.length()==0&&(s.charAt(i)==')'||s.charAt(i)=='}'||s.charAt(i)==']'))retu
4000
rn
<span style="white-space:pre">			</span>// false;
<span style="white-space:pre">			</span>if (s.charAt(i) == '(')
<span style="white-space:pre">				</span>str += '(';
<span style="white-space:pre">			</span>if (s.charAt(i) == ')') {
<span style="white-space:pre">				</span>if (str.length() == 0 || str.charAt(str.length() - 1) != '(')
<span style="white-space:pre">					</span>return false;
<span style="white-space:pre">				</span>else
<span style="white-space:pre">					</span>str = str.substring(0, str.length() - 1);
<span style="white-space:pre">			</span>}
<span style="white-space:pre">			</span>if (s.charAt(i) == '[')
<span style="white-space:pre">				</span>str += '[';
<span style="white-space:pre">			</span>if (s.charAt(i) == ']') {
<span style="white-space:pre">				</span>if (str.length() == 0 || str.charAt(str.length() - 1) != '[')
<span style="white-space:pre">					</span>return false;
<span style="white-space:pre">				</span>else
<span style="white-space:pre">					</span>str = str.substring(0, str.length() - 1);
<span style="white-space:pre">			</span>}
<span style="white-space:pre">			</span>if (s.charAt(i) == '{')
<span style="white-space:pre">				</span>str += '{';
<span style="white-space:pre">			</span>if (s.charAt(i) == '}') {
<span style="white-space:pre">				</span>if (str.length() == 0 || str.charAt(str.length() - 1) != '{')
<span style="white-space:pre">					</span>return false;
<span style="white-space:pre">				</span>else
<span style="white-space:pre">					</span>str = str.substring(0, str.length() - 1);
<span style="white-space:pre">			</span>}
<span style="white-space:pre">		</span>}
<span style="white-space:pre">		</span>if (str.length() == 0)
<span style="white-space:pre">			</span>return true;
<span style="white-space:pre">		</span>return false;
<span style="white-space:pre">	</span>}
}