A bug on bubble sorting

A bug on bubble sorting

I want to sort a 2*n matrix, n is given in the input. Make a program to
output a matrix . Here is the requirment. (1)the first column MUST be
sorted in ASC, and (2)the second column in DESC if possible.
For Example, let n = 5, and the matrix is
3 4
1 2
3 5
1 6
7 3
The result should be
1 6
1 2
3 5
3 4
7 3
So I write down the code like this. First Line input the value n , and the
following lines like above.
#include <stdio.h>
#define TWO_16 65536
#define TWO_15 32768
int v[100000][2];
int z[100000];
int vec[100000];
int n;
int main()
{
int i, j;
scanf ("%d", &n); // give the value of n;
for (i = 1; i <= n; i++) // filling the matrix;
{
scanf ("%d%d", &v[i][0], &v[i][1]);
z[i] = TWO_16 * v[i][0] + TWO_15 - v[i][1];
vec[i] = i;
}
for (i = 1; i <= n; i++)
for (j = 1; j <= i; j++)
{
if (z[j] > z[i])
{
int t = vec[i];
vec[i] = vec[j];
vec[j] = t;
}
}
for (i = 1; i <= n; i++) // output the matrix
printf("%d %d\n",v[vec[i]][0],v[vec[i]][1]);
return 0;
}
But in gcc, the output is
1 6
3 5
3 4
1 2
7 3
What's more, when the first line is changed to "1 2" and the second is
changed to "3 4" in input, the result also changed.
What's the problem of my code?