20.c

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

typedefintbool;
#definetrue 1
#definefalse 0

structNode {
charval;
structNode*next;
};

voidpush(structNode**top_pt, charnew_data)
{
structNode*new_node= (structNode*)malloc(sizeof(structNode));
new_node->val=new_data;
new_node->next=*top_pt;

*top_pt=new_node;
}

intpop(structNode**top_pt)
{
if (*top_pt==NULL)
 {
printf("stack overflow\n");
exit(0);
 }
structNode*top=*top_pt;
charres=top->val;
*top_pt=top->next;
free(top);
returnres;
}

boolisValid(char*s) {
structNode*stack=NULL;
inti;
intlen=strlen(s);
for (i=0; i<len; i++) {
if (s[i] =='('||s[i] =='['||s[i] =='{') {
push(&stack, s[i]);
 }
else {
if (stack==NULL)
return false;
chartop=pop(&stack);
if (s[i] ==')') {
if (top!='(') return false;
 }
elseif (s[i] ==']'){
if (top!='[') return false;
 }
elseif (s[i] =='}'){
if (top!='{') return false;
 }
 }
 }
if (stack==NULL)
return true;
else
return false;
}

voidprint_stack(structNode**top_pt)
{
structNode*t=*top_pt;
while (t!=NULL)
 {
structNode*tmp_node=t;
printf("%c ", tmp_node->val);
t=tmp_node->next;
 }
printf("\n");
}

intmain()
{
chars[] ="]";
printf("%d\n", isValid(s));
return0;
}