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.

維持一個字元棧和位置棧，字元棧記錄沒有匹配的字元，位置棧記錄每一個入棧的字元的位置，如果入棧的是右括號，並且棧頂為左括號，則退棧，求出當前連續退棧的字元長度，求出這個最大值就是。

左括號入棧和沒有匹配的右括號入棧

int longestValidParentheses(char* s) {
    int tmp=0,i=0, max=0;
    int s_length=strlen(s);
    
    char* s_stack=malloc(sizeof(char)*s_length);
    int* number_stack=malloc(sizeof(int)*s_length);
    
    int s_stack_index=0;

    while(i<s_length){
        if(s[i]=='('){
            s_stack[s_stack_index]=s[i];
            number_stack[s_stack_index]=i;
            s_stack_index++;

        } else {
                    if (s_stack_index>=1 && s_stack[s_stack_index-1]=='('){
                        s_stack_index--;
                        if(s_stack_index==0) tmp=i+1;   // 棧裡的出棧出完了的話，說明出棧完
                        else tmp=i-number_stack[s_stack_index-1];
                        if(tmp>max) max=tmp;
                    } else {
                        s_stack[s_stack_index]=s[i];
                        number_stack[s_stack_index]=i;
                        s_stack_index++;
                    }
            
        }
        
        i++;
    }
    
    
    free(s_stack);
    free(number_stack);
    return max;
    
}