780. Reaching Points

idea:

1.从后向前找

2.while (tx > ty) tx -= ty; 替为 % 操作

3.经过循环后，必定只有两种情况才true

1. sx == tx && sy <= ty && (ty - sy) % sx == 0
2. sy == ty && sx <= tx && (tx - sx) % sy == 0

public boolean reachingPoints(int sx, int sy, int tx, int ty) {
        while (tx > sx && ty > sy){
            if (tx > ty)    tx %= ty;
            else    ty %= tx;
        }
        return sx == tx && sy <= ty && (ty - sy) % sx == 0 ||
            sy == ty && sx <= tx && (tx - sx) % sy == 0;

    }