Lesson 5 : Brackets (Brackets)

Lesson 5 : Brackets
https://codility.com/programmers/lessons/5

This is a simple problem to check the matchings of brackets. 
Here is the simplest solution. 

Simply, we put the left bracket characters onto the stack when any of them is found. When any right bracket is found, we check if the character (left bracket) on the stack top matches to the right bracket.

One thing that should not be forgotten is see if the string is correctly terminated with no left bracket on the stack.

This solution gives the 100% score.

#include <alloca.h>

int solution(char *S) {
    
    int len = strlen(S);
    
    if (len == 0){
        return 1;
    }
    
    //I always wonder why thye don't tell anything about
    //what we shoud do when memory allocation failed 
    //in codility problems...
    char*   stack = (char*)alloca(len);
  
  
    int idx = 0;  //this is a stack pointer.
    
    int i;
    //scan the character one-by-one
    for (i = 0; i < len; i++){
        char c = S[i];
        
        switch(c){
            //if the character is '(', '[', '{',
            //just push it onto the stack.
            case '(':
            case '[':
            case '{':
                stack[idx] = c;
                idx++;
                break;
                
            //if the character is '(', '[', '{',
            //check if the last character matches with each
            case ')':
                idx--;
                if (stack[idx] != '('){
                    return 0;
                }
                break;
            case ']':
                idx--;
                if (stack[idx] != '['){
                    return 0;
                }
                break;
            case '}':
                idx--;
                if (stack[idx] != '{'){
                    return 0;
                }
                break;
            default:
                return 0;
        }
    }
    
    //we need to see if the string is terminted collectly
    if (idx != 0){
        return 0;
    }
    
    return 1;
}