# Leetcode 1720. Decode XORed Array
###### tags: `Leetcode`

題目
There is a hidden integer array arr that consists of n non-negative integers.

It was encoded into another integer array encoded of length n - 1, such that encoded[i] = arr[i] XOR arr[i + 1]. For example, if arr = [1,0,2,1], then encoded = [1,2,3].

You are given the encoded array. You are also given an integer first, that is the first element of arr, i.e. arr[0].

Return the original array arr. It can be proved that the answer exists and is unique.

Example 1:

Input: encoded = [1,2,3], first = 1
Output: [1,0,2,1]
Explanation: If arr = [1,0,2,1], then first = 1 and encoded = [1 XOR 0, 0 XOR 2, 2 XOR 1] = [1,2,3]
Example 2:

Input: encoded = [6,2,7,3], first = 4
Output: [4,2,0,7,4]

解法:
1.只要知道若a xor b = c，則abc三者兩兩xor就會等於另外一個就好

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/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
int* decode(int* encoded, int encodedSize, int first, int* returnSize){
    *returnSize = encodedSize+1;
    int* res = malloc(sizeof(int) * *returnSize);
    res[0] = first;
    for(int i = 1; i < *returnSize; i++)
        res[i] = res[i-1]^encoded[i-1];
    return res;
}