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// 421. Maximum XOR of Two Numbers in an Array
#include "leetcode.h"
/*
* given an integer array 'nums', return the maximum result of 'nums[i] ^
* nums[j]' where '0 <= i <= j < n'
*/
struct tree {
struct tree *left;
struct tree *right;
};
static int ans;
struct tree *root;
struct tree *new_node(void) {
struct tree *obj = (struct tree *)malloc(sizeof(struct tree));
obj->left = NULL;
obj->right = NULL;
return obj;
}
void trie_create(struct tree *r, int n, int width) {
if (width <= 0 || !r)
return;
if (n & (1 << (width - 1))) {
if (!r->right)
r->right = new_node();
trie_create(r->right, n, width - 1);
} else {
if (!r->left)
r->left = new_node();
trie_create(r->left, n, width - 1);
}
return;
}
void max_xor(struct tree *high, struct tree *low, int xor, int bit_pos) {
bool skip = false;
if (!low) {
ans = ans < xor ? xor : ans;
return;
}
if (high->right && low->left) {
xor |= 1 << (bit_pos - 1);
max_xor(high->right, low->left, xor, bit_pos - 1);
xor &= ~(1 << (bit_pos - 1));
skip = true;
}
if (high->left && low->right) {
xor |= 1 << (bit_pos - 1);
max_xor(high->left, low->right, xor, bit_pos - 1);
xor &= ~(1 << (bit_pos - 1));
skip = true;
}
if (!skip) {
max_xor(high->left, low->left, xor, bit_pos - 1);
max_xor(high->right, high->right, xor, bit_pos - 1);
}
}
int findMaximumXOR(int *nums, int numsSize) {
struct tree *node = root;
int num, max = -1, bit_pos = 0, xor = 0;
for (int i = 0; i < numsSize; i++)
max = max < nums[i] ? nums[i] : max;
while (max) {
bit_pos++;
max >>= 1;
}
root = new_node();
for (int i = 0; i < numsSize; i++)
trie_create(root, nums[i], bit_pos);
struct tree *high, *low;
high = low = root;
ans = 0;
max_xor(root, root, 0, bit_pos);
return ans;
}
int main() {
int n1[] = {3, 10, 5, 25, 2, 8},
n2[] = {14, 70, 53, 83, 49, 91, 36, 80, 92, 51, 66, 70};
printf("%d\n", findMaximumXOR(n1, ARRAY_SIZE(n1))); // expect: 28
printf("%d\n", findMaximumXOR(n2, ARRAY_SIZE(n2))); // expect: 127
}
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