Missing Number

Given an array containing n distinct numbers taken from 0, 1, 2, ..., n, find the one that is missing from the array.

For example,
Given nums = [0, 1, 3] return 2.

Note:
Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

public class Solution {

    public int missingNumber(int[] nums) {

        if (nums == null || nums.length == 0) {

            return 0;

        }

        for (int i = 0; i < nums.length; i++) {

            int n = nums[i];

            if (n < nums.length && nums[n] != n) {

                nums[i] = nums[n];

                nums[n] = n;

                i--;

            }

        }

        for (int i = 0; i < nums.length; i++) {

            if (i != nums[i]) {

                return i;

            }

        }

        return nums.length;

    }

}

2.Bitwise operation solution

public int MissingNumber(int[] nums) {
    int xorResult = 0;
    for(int i = 0; i < nums.Length; i++)
        xorResult ^= nums[i] ^ i;
    xorResult ^= nums.Length;
    return xorResult;
}

3.Math solution by sum total

public int MissingNumber(int[] nums) {
    int result = nums.Length * (nums.Length + 1) / 2;
    for(int i = 0; i < nums.Length; i++)
        result -= nums[i];
    return result;
}