268. Missing Number (Easy)

https://leetcode.com/problems/missing-number/

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

Example 1:

Input: [3,0,1]
Output: 2

Example 2:

Input: [9,6,4,2,3,5,7,0,1]
Output: 8

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

Solutions

class Solution {
    public int missingNumber(int[] nums) {
        if (nums == null || nums.length == 0) {
            return -1;
        }

        int n = nums.length;

        // n is the special case, check out whether it exists or not.
        boolean nExists = false;
        for (int i = 0; i < n; i++) {
            if (nums[i] == n) {
                nExists = true;
                break;
            }
        }

        // If not present, return it immediately.
        if (!nExists) {
            return n;
        }

        for (int i = 0; i < n; i++) {
            if (nums[i] == n) {
                continue;
            }

            if (nums[i] != i) {
                // swap nums[i] to its corresponding slot
                swap(nums, nums[i], i);

                // do not move forward until current position is settled
                i--;
            }
        }

        // Finally, the slot of missing number will be swapped in n
        for (int i = 0; i < n; i++) {
            if (nums[i] != i) {
                return i;
            }
        }

        return -1;
    }

    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}

Incorrect Solutions

References

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