how to sort vector of points based on X axis in opencv using c?

suppose i have two vectors. one contain data in ascending order(0,106),(0,107),(1,5),(1,6),(2,8).... another vector has points (0,2),(0,5),(0,106),(0,107),(0,108),(1,6),(2,1).....and so on.. I want output as (0,2),(0,5),(0,106),(0,107),(0,108),(1,5),(1,6),(2,1),(2,8). here the code:

 float   x1[1500],y1[1500],x2[1500],y2[1500];
    float x3[2500],y3[2500];
    int ik=0,kk=0;

    int p1=0;

int j = 0;
k = 0;
int i=0;

for(int o=0;o<l+l1;)
{
    if(i<l && j<l1)
{
    if(x1[i]==x2[j])
    {
        if(y1[i]==y2[j])
        {
            x3[p1]=x1[i];

            y3[p1]=y1[i];

            p1++;
            i++;
            j++;
        }

        else if(y1[i]<y2[j])
        {
            x3[p1]=x1[i];
            y3[p1]=y1[i];

            p1++;
            i++;}

        else
            {
            x3[p1]=x2[j];
            y3[p1]=y2[j];
            fs1<<"=x3="<<x3[p1]<<" y3="<<y3[p1]<<endl;
            p1++;
            j++;}


    }
    else if(x1[i]<x2[j])
    {
        x3[p1]=x1[i];
            y3[p1]=y1[i];

            p1++;
            i++;

    }
    else
    {
        x3[p1]=x2[j];
            y3[p1]=y2[j];

            p1++;
            j++;

    }o++;
}
if(i==l && j<l1)
    {
        x3[p1]=x2[j];
            y3[p1]=y2[j];

            p1++;
            j++;
            o++;
    }


if(i<l && j==l1)
            {
                x3[p1]=x1[i];
                y3[p1]=y1[i];
                p1++;
                i++;
o++;}

    }

Thank you in advance