1.Given an array and a value, 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:
Given nums = [3,2,2,3], val = 3,
Your function should return length = 2, with the first two elements of nums being
2.题目要求：移除一个数组中和给定值相同的数字，并返回新的数组的长度。
3.方法：只需要一个变量用来计数，然后遍历原数组，如果当前的值和给定值不同，我们就把当前值覆盖计数变量的位置，并将计数变量加1。
4.代码：
class Solution {
public:
int removeElement(vector<int>& nums, int val) {
int res = 0;
for (int i = 0; i < nums.size(); ++i) {
if (nums[i] != val) nums[res++] = nums[i];
}
return res;
}
};
