CareerCup

Assuming all numbers in the set are positive.
Step 1. Add the numbers in the set.
Step 2. Subtract the given sum from added total at step1.
Step 3. Divide number by 2 at step 2. Reverse the sign of this particular number.

O(n) time, O(1) space

static int reverseSignElement(int[] array, int target) {
		if (array == null || array.length == 0)
			return 0;
		int sum = 0;
		for (int i : array) {
			sum += i;
		}
		for (int i : array) {
			if (sum - 2*i == target)
				return i;
		}
		return 0;
	}

logic derivation:

Change of sign a element in set will return the target number.

Assume R is the resultant element and we will change the sign of R

So in the sum of the all elements of set (Say Sum), R is also included. First exclude that.
So we have sum of all elements except R => Sum – R.
Now, we changed sign of R and add it to get new sum of all elements.
( Sum –R ) + ( –R) = target //we changed sign of R
i.e, Sum –R –R = target
i.e, Sum – 2*R = target

Now for which i in set, this equation will satisfy, that is the result.