题目
Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.
For "(()", the longest valid parentheses substring is "()", which has length = 2.
Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.
分析
寻找一个子串，包含最长的有效圆括号对。可以通过栈来统计一共出现了多少对圆括号。但是要找最长子字符串，就需要进一步分析。
一个子函数用来判断后续的前缀字符串是否圆括号匹配，如果匹配就可以与前面的子串相加，否则就是新的子串，重新判断最大长度。
int validSubstring(char *s,int length)
{
int left=0;
while(s[length]!='\0')
{
if(s[length]==')')
{
if(left<=0)return -1;
else if(left==1)
return 1;
else
left--;
}
else
{
left++;
}
length++;
}
if(left>0)return -1;
else return 1;
}
int longestValidParentheses(char* s) {
int ans=0,temp=0,length=0,left=0,lastleft=0;
while(s[length]!='\0')
{
if(s[length]=='(')
{
if(validSubstring(s,length)==-1)
{
temp=0;
left=1;
}
else
left++;
}
else
{
if(left>0)
{
left--;
temp++;
if(temp>ans)ans=temp;
}
else
{
left=0;
temp=0;
}
}
//printf("%d %d\n",left,temp);
length++;
}
return ans*2;
}
