27. Remove Element #

Problem #

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.

Clarification:

Confused why the returned value is an integer but your answer is an array?

Note that the input array is passed in by reference, which means modification to the input array will be known to the caller as well.

Internally you can think of this:

// nums is passed in by reference. (i.e., without making a copy)
int len = removeElement(nums, val);

// any modification to nums in your function would be known by the caller.
// using the length returned by your function, it prints the first len elements.
for (int i = 0; i < len; i++) {
    print(nums[i]);
}

Problem Summary #

Given an array nums and a value val, remove all elements in the array that are equal to val, and return the number of remaining elements.

Solution Approach #

This problem is very similar to problem 283. This problem is basically the same as problem 283: problem 283 removes 0, while this problem removes a given val. In essence, they are the same.

Here, deletion from the array is not actual deletion. Instead, the elements to be deleted are moved to the space at the end of the array, and then the actual number of remaining elements in the array is returned. When the OJ ultimately judges the problem, it will read and output the remaining number of elements in the array.

Code #

package leetcode

func removeElement(nums []int, val int) int {
	if len(nums) == 0 {
		return 0
	}
	j := 0
	for i := 0; i < len(nums); i++ {
		if nums[i] != val {
			if i != j {
				nums[i], nums[j] = nums[j], nums[i]
			}
			j++
		}
	}
	return j
}