有两个有序整数集合a和b，写一个函数找出它们的交集？

　private static Set<Integer> setMethod(int[] a,int[] b){
Set<Integer> set = new HashSet<Integer>();
Set<Integer> set2 = new HashSet<Integer>();
for(int i=0; i<a.length; i++) {
set.add(a[i]);
}
for(int j=0; j<b.length; j++) {
if(!set.add(b[j]))
set2.add(b[j]);
}
return set2;
}

private static Set<Integer> forMethod(int[] a,int[] b){
Set<Integer> set=new HashSet<Integer>();
int i=0,j=0;
while(i<a.length && j<b.length){
if(a[i]<b[j])
i++;
else if(a[i]>b[j])
j++;
else{
set.add(a[i]);
i++;
j++;
}
}
return set;
}

private static int[] intersect(int[] a, int[] b) {
if (a[0] > b[b.length - 1] || b[0] > a[a.length - 1]) {
return new int[0];
}
int[] intersection = new int[Math.max(a.length, b.length)];
int offset = 0;
for (int i = 0, s = i; i < a.length && s < b.length; i++) {
while (a[i] > b[s]) {
s++;
}
if (a[i] == b[s]) {
intersection[offset++] = b[s++];
}
while (i < (a.length - 1) && a[i] == a[i + 1]) {
i++;
}
}
if (intersection.length == offset) {
return intersection;
}
int[] duplicate = new int[offset];
System.arraycopy(intersection, 0, duplicate, 0, offset);
return duplicate;
}

public class NumberCrossTest {
public static void main(String[] args) {
int[] a1 = new int[100000];
for (int i = 0; i < a1.length; i++) {
a1[i] = i + 10;
}
int[] a2 = new int[200000];
for (int i = 0; i < a2.length; i++) {
a2[i] = i + 20;
}
long begin = System.currentTimeMillis();
Set<Integer> set1 = setMethod(a1, a2);
long end = System.currentTimeMillis();
System.out.println(end - begin);// 359
begin = System.currentTimeMillis();
Set<Integer> set2 = forMethod(a1, a2);
end = System.currentTimeMillis();
System.out.println(end - begin);// 160
begin = System.currentTimeMillis();
int[] c = intersect(a1, a2);
end = System.currentTimeMillis();
System.out.println(end - begin);// 10
// 测试两种方法的结果是否相等
System.out.println(set1.equals(set2));// true
Set<Integer> set3 = new HashSet<Integer>();
for (int i = 0; i < c.length; i++) {
set3.add(c[i]);
}
System.out.println(set1.equals(set3));// true
}

private static Set<Integer> setMethod(int[] a, int[] b) {
Set<Integer> set = new HashSet<Integer>();
Set<Integer> set2 = new HashSet<Integer>();
for (int i = 0; i < a.length; i++) {
set.add(a[i]);
}
for (int j = 0; j < b.length; j++) {
if (!set.add(b[j]))
set2.add(b[j]);
}
return set2;
}

private static Set<Integer> forMethod(int[] a, int[] b) {
Set<Integer> set = new HashSet<Integer>();
int i = 0, j = 0;
while (i < a.length && j < b.length) {
if (a[i] < b[j])
i++;
else if (a[i] > b[j])
j++;
else {
set.add(a[i]);
i++;
j++;
}
}
return set;
}

private static int[] intersect(int[] a, int[] b) {
if (a[0] > b[b.length - 1] || b[0] > a[a.length - 1]) {
return new int[0];
}
int[] intersection = new int[Math.max(a.length, b.length)];
int offset = 0;
for (int i = 0, s = i; i < a.length && s < b.length; i++) {
while (a[i] > b[s]) {
s++;
}
if (a[i] == b[s]) {
intersection[offset++] = b[s++];
}
while (i < (a.length - 1) && a[i] == a[i + 1]) {
i++;
}
}
if (intersection.length == offset) {
return intersection;
}
int[] duplicate = new int[offset];
System.arraycopy(intersection, 0, duplicate, 0, offset);
return duplicate;
}
}