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[leetcode] problem 27 – remove element

admin 11月 13, 2020 0

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

No.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.

No.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:

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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]);
}

Code

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public int (int[] nums, int val) {
    if (nums == null || nums.length < 1)
        return 0;

    int length = 0;

    for (int i = 0; i < nums.length; i++) {
        if (nums[i] != val)
            nums[length++] = nums[i];
    }

    return length;
}

When elements to remove are rare

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public int (int[] nums, int val) {
    if (nums == null || nums.length < 1)
        return 0;

    int start = 0;
    int end = nums.length - 1;

    while (start <= end) {
        if (nums[start] == val)
            nums[start] = nums[end--];
        else
            start++;
    }

    return end + 1;
}
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