Leetcode 27. Remove Element-求出不等于val的元素个数，还需要交换元素，需要前几个元素都不等于val，等于val的元素要放在数组后面

Given an array nums and a value val, remove all instances of that value in-place and return the new length.

Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory.

The order of elements can be changed. It doesn't matter what you leave beyond the new length.

Example 1:

Given nums = [3,2,2,3], val = 3,
Your function should return length = 2, with the first two elements o