136. Single Number

Given an array of integers, every element appears twice except for one. Find that single one.

Concept

• If we take XOR of zero and some bit, it will return that bit
  • a ⊕ 0 = a
• If we take XOR of two same bits, it will return 0
  • a ⊕a = 0
• a ⊕b⊕a = (a ⊕ a) ⊕ b = 0⊕b = b

So we can XOR all bits together to find the unique number.

class Solution {
    public int singleNumber(int[] nums) {
        int tmp=0;
        for(int num:nums){
            tmp^=num;
        }
        return tmp;
    }
}

389. Find the Difference

Given two strings s

String t is generated by random shuffling string s and then add one more letter at a random position.

Find the letter that was added in t.

Example:

Input:
s = "abcd"
t = "abcde"

Output:
e

Explanation:
'e' is the letter that was added.