27. Remove Element

Given an integer array nums and an integer val, remove all occurrences of val in nums in-place. The order of the elements may be changed. Then return the number of elements in nums which are not equal to val.

Consider the number of elements in nums which are not equal to val be k, to get accepted, you need to do the following things:

Change the array nums such that the first k elements of nums contain the elements which are not equal to val. The remaining elements of nums are not important as well as the size of nums. Return k.

Example 1:

Input: nums = [3,2,2,3], val = 3

Output: k = 2, nums = [2,2,_,_]

Example 2:

Input: nums = [0,1,2,2,3,0,4,2], val = 2

Output: k = 5, nums = [0,1,3,0,4,_,_,_]

Custom Judge:

The judge will test your solution with the following code:

int[] nums = [...]; // Input array
int val = ...; // Value to remove
int[] expectedNums = [...]; // The expected answer with correct length.
                            // It is sorted with no values equaling val.

int k = removeElement(nums, val); // Calls your implementation

assert k == expectedNums.length;
sort(nums, 0, k); // Sort the first k elements of nums
for (int i = 0; i < k; i++) {
    assert nums[i] == expectedNums[i];
}

Swap-with-last approach:

i scans forward from the start, n tracks the current logical end of the array. When nums[i] == val, shrink n by one and overwrite nums[i] with whatever sits at the new last index nums[n], then re-check the same i (the swapped-in value hasn’t been checked yet). When nums[i] != val, it’s already in the “keep” region, so just advance i. Elements equal to val end up pushed past index n, and the loop only ever touches each index once as either the scanning pointer or the source of a swap, giving O(k) time (k = nums.len()) and O(1) extra space.

#![allow(unused)]
fn main() {
pub fn remove_element(nums: &mut Vec<i32>, val: i32) -> i32 {
    let mut i = 0;
    let mut n = nums.len();
    while i < n {
        if nums[i] == val {
            n -= 1;
            nums[i] = nums[n];
        } else {
            i += 1;
        }
    }

    n as i32
}
}