27. Remove Element

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 of nums being 2.
It doesn't matter what you leave beyond the returned length.

Example 2:
Given nums = [0,1,2,2,3,0,4,2], val = 2,
Your function should return length = 5, with the first five elements of nums containing 0, 1, 3, 0, and 4.
Note that the order of those five elements can be arbitrary.
It doesn't matter what values are set beyond the returned length.

难度：easy

题目：
给定整数数组和一个特定值，原地移除所有给定的特定值实例，并且返回新数组长度。
不要申请额外的空间，必须在给出的数组上原地改动。
数组元素所在位置可以被改动。数组实际长度可以超过新数组长度。

思路：从后向前转换

Runtime: 4 ms, faster than 100.00% of Java online submissions for Remove Element.
Memory Usage: 26.9 MB, less than 13.46% of Java online submissions for Remove Element.

class Solution {
    public int removeElement(int[] nums, int val) {
        int idx = nums.length - 1;
        for (int i = 0; i <= idx; i++) {
            if (nums[i] == val) {
                if (nums[i] == nums[idx]) {
                    i--;
                } else {
                    nums[i] = nums[idx];
                }
                idx--;
            }
        }
        
        return idx + 1;
    }
}