Given an array and a value, remove all instances of that value in place and return the new length.
The order of elements can be changed. It doesn't matter what you leave beyond the new length.
遍历数组，并逐一原地拷贝不等于key的元素
int removeElement(int a[], int n, int key)
{
int k = 0;
for (int i = 0; i < n; ++i) {
if (a[i] != key)
a[k++] = a[i];
}
return k;
}