a+b=c

// Let a,b and c be 3-digit numbers such that every digit 1 to 9
// occurs exactly once if all number are written in concatenation.
// For which number is a+b=c true.
// This is a riddle from a mathematics-book for fourth-graders. It is marked
// as "difficult, for students that are faster than others".


var buffer = [0,0,0,0,0,0,0,0,0];

function add(pos, callback) {
    if (pos<0) {
        callback(buffer);
        return;
    }
    for(var i=1; i<=9;i++) {
        if (buffer.indexOf(i,pos)<0) {
            buffer[pos]=i;
            add(pos-1,callback);
        }
    }
}

add(8,function(permutation) {
    var a = permutation[0]*100 + permutation[1]*10 + permutation[2];
    var b = permutation[3]*100 + permutation[4]*10 + permutation[5];
    var c = permutation[6]*100 + permutation[7]*10 + permutation[8];
    if (a+b==c) {
        console.log(a+" + "+b+" = "+c);
    }
});