Gray Code https://leetcode.com/explore/item/3799

grayCode.c

/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
int* grayCode(int n, int* returnSize) {
    int size = 1 << n; // 2 ^ n
    int *sequence = (int *) calloc(size, sizeof(int));
    
    sequence[0] = 0;
    sequence[1] = 1;
    
    for (int bitcount = 2; bitcount <= n; bitcount++) {
        int curSeqSize = 1 << bitcount;
        int lastSeqSize = curSeqSize / 2;
        // first half: i -> (i << 1)
        // Ex. bitcount = 2
        // 1) 0 -> 00
        // 2) 1 -> 10
        for (int i = 0; i < lastSeqSize; i++) {
            // printf("%d) %d -> ", i + 1, sequence[i]);
            sequence[i] <<= 1;
            // printf("%d\n", sequence[i]);
        }
        
        // second half: i -> (i << 1 + 1) in reverse order
        // Ex. bitcount = 2
        // 3) 1 -> 10 + 1 -> 11
        // 4) 0 -> 00 + 1 -> 01
        for (int i = 0; i < lastSeqSize; i++) {
            // printf("%d) %d -> ", lastSeqSize + i + 1, sequence[lastSeqSize - i - 1]);
            sequence[lastSeqSize + i] = sequence[lastSeqSize - i - 1] + 1;
            // printf("%d\n", sequence[lastSeqSize + i]);
        }
    }
    *returnSize = size;
    
    return sequence;
}

https://leetcode.com/submissions/detail/516069861/