# Find maximum XOR of given integer in a stream of integers

You are given a number of queries **Q** and each query will be of the following types:

**Query 1**: add(x) This means add x into your data structure.**Query 2**: maxXOR(y) This means print the maximum possible XOR of y with all the elements already stored in the data structure.

1 <= x, y <= 10^9

1 <= 10^5 <= Q

The data structure begins with only a 0 in it.

Example:

Input: (1 10), (1 13), (210), (1 9), (1 5), (26) Output: 7 15 Add 10 and 13 to stream. Find maximum XOR with 10, which is 7 Insert 9 and 5 Find maximum XOR with 6 which is 15.

A good way to solve this problem is to use a Trie. A **prerequisite** for this post is Trie Insert and Search.

Each Trie Node will look following:

struct TrieNode { // We use binary and hence we // need only 2 children TrieNode* Children[2]; bool isLeaf; };

Another thing to handle is that we have to pad the binary equivalent of each input number by a suitable number of zeros to the left before storing them. The maximum possible value of x or y is 10^9 and hence 32 bits will be sufficient.

So how does this work?

Assume we have to insert 3 and 7 into Trie. The Trie starts out with 0 and after these three insertions can be visualized like this:

For simplification, the padding has been done to store each number using 3 bits. Note that in binary:

3 is 011

7 is 111

Now if we have to insert 1 into our Trie, we can note that 1 is 001 and we already have path for 00. So we make a new node for the last set bit and after connecting, we get this:

Now if we have to take XOR with 5 which is 101, we note that for the leftmost bit (position 2), we can choose a 0 starting at the root and thus we go to the left. This is the position 2 and we add 2^2 to the answer.

For position 1, we have a 0 in 5 and we see that we can choose a 1 from our current node. Thus we go right and add 2^1 to the answer.

For position 0, we have a 1 in 5 and we see that we cannot choose a 0 from our current node, thus we go right.

The path taken for 5 is shown above. The answer is thus 2^2 + 2^1 = 6.

`// C++ program to find maximum XOR in` `// a stream of integers` `#include<bits/stdc++.h>` `using` `namespace` `std;` ` ` `struct` `TrieNode` `{` ` ` `TrieNode* children[2];` ` ` `bool` `isLeaf;` `};` ` ` `// This checks if the ith position in` `// binary of N is a 1 or a 0` `bool` `check(` `int` `N, ` `int` `i)` `{` ` ` `return` `(` `bool` `)(N & (1<<i));` `}` ` ` `// Create a new Trie node` `TrieNode* newNode()` `{` ` ` `TrieNode* temp = ` `new` `TrieNode;` ` ` `temp->isLeaf = ` `false` `;` ` ` `temp->children[0] = NULL;` ` ` `temp->children[1] = NULL;` ` ` `return` `temp;` `}` ` ` `// Inserts x into the Trie` `void` `insert(TrieNode* root, ` `int` `x)` `{` ` ` `TrieNode* Crawler = root;` ` ` ` ` `// padding upto 32 bits` ` ` `for` `(` `int` `i = 31; i >= 0; i--)` ` ` `{` ` ` `int` `f = check(x, i);` ` ` `if` `(! Crawler->children[f])` ` ` `Crawler->children[f] = newNode();` ` ` `Crawler = Crawler->children[f];` ` ` `}` ` ` `Crawler->isLeaf = ` `true` `;` `}` ` ` `// Finds maximum XOR of x with stream of` `// elements so far.` `int` `query2(TrieNode *root, ` `int` `x)` `{` ` ` `TrieNode* Crawler = root;` ` ` ` ` `// Do XOR from root to a leaf path` ` ` `int` `ans = 0;` ` ` `for` `(` `int` `i = 31; i >= 0; i--)` ` ` `{` ` ` `// Find i-th bit in x` ` ` `int` `f = check(x, i);` ` ` ` ` `// Move to the child whose XOR with f` ` ` `// is 1.` ` ` `if` `((Crawler->children[f ^ 1]))` ` ` `{` ` ` `ans = ans + (1 << i); ` `// update answer` ` ` `Crawler = Crawler->children[f ^ 1];` ` ` `}` ` ` ` ` `// If child with XOR 1 doesn't exist` ` ` `else` ` ` `Crawler = Crawler->children[f];` ` ` `}` ` ` ` ` `return` `ans;` `}` ` ` `// Process x (Add x to the stream)` `void` `query1(TrieNode *root, ` `int` `x)` `{` ` ` `insert(root, x);` `}` ` ` `// Driver code` `int` `main()` `{` ` ` `TrieNode* root = newNode();` ` ` `query1(root, 10);` ` ` `query1(root, 13);` ` ` `cout << query2(root, 10) << endl;` ` ` `query1(root, 9);` ` ` `query1(root, 5);` ` ` `cout << query2(root, 6) << endl;` ` ` `return` `0;` `}` |

7 15

The space taken by the Trie is O(n*log(n)). Each query of type 1 takes O(log(n)) time. Each query of type 2 takes O(log(n)) time too. Here n is the largest query number.

**Follow up problem:** What if we are given three queries instead of two?

1) add(x) This means add x into your data structure (duplicates are allowed).

2) maxXOR(y) This means print the maximum possible XOR of y with all the elements already stored in the data structure.

3) remove(z) This means remove one instance of z from the data structure.

What changes in the Trie solution can achieve this?

This article is contributed by **Hemang Sarkar**. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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