题目：

题解：

const int HIGH_BIT = 30;

struct Trie {
    // 左子树指向表示 0 的子节点
    struct Trie* left;
    // 右子树指向表示 1 的子节点
    struct Trie* right;
};

struct Trie* createTrie() {
    struct Trie* ret = malloc(sizeof(struct Trie));
    ret->left = ret->right = NULL;
    return ret;
}

void add(struct Trie* root, int num) {
    struct Trie* cur = root;
    for (int k = HIGH_BIT; k >= 0; --k) {
        int bit = (num >> k) & 1;
        if (bit == 0) {
            if (!cur->left) {
                cur->left = createTrie();
            }
            cur = cur->left;
        } else {
            if (!cur->right) {
                cur->right = createTrie();
            }
            cur = cur->right;
        }
    }
}

int check(struct Trie* root, int num) {
    struct Trie* cur = root;
    int x = 0;
    for (int k = HIGH_BIT; k >= 0; --k) {
        int bit = (num >> k) & 1;
        if (bit == 0) {
            // a_i 的第 k 个二进制位为 0，应当往表示 1 的子节点 right 走
            if (cur->right) {
                cur = cur->right;
                x = x * 2 + 1;
            } else {
                cur = cur->left;
                x = x * 2;
            }
        } else {
            // a_i 的第 k 个二进制位为 1，应当往表示 0 的子节点 left 走
            if (cur->left) {
                cur = cur->left;
                x = x * 2 + 1;
            } else {
                cur = cur->right;
                x = x * 2;
            }
        }
    }
    return x;
}

int findMaximumXOR(int* nums, int numsSize) {
    struct Trie* root = createTrie();
    int x = 0;
    for (int i = 1; i < numsSize; ++i) {
        // 将 nums[i-1] 放入字典树，此时 nums[0 .. i-1] 都在字典树中
        add(root, nums[i - 1]);
        // 将 nums[i] 看作 ai，找出最大的 x 更新答案
        x = fmax(x, check(root, nums[i]));
    }
    return x;
}