[LeetCode] (medium) 421. Maximum XOR of Two Numbers in an Array

https://leetcode.com/problems/maximum-xor-of-two-numbers-in-an-array/

Given a non-empty array of numbers, a0, a1, a2, … , an-1, where 0 ≤ ai < 231.

Find the maximum result of ai XOR aj, where 0 ≤ i, j < n.

Could you do this in O(n) runtime?

Example:

Input: [3, 10, 5, 25, 2, 8]
Output: 28
Explanation: The maximum result is 5 ^ 25 = 28.

---

字典树：

 

class Solution {
public:
    struct Node{
        Node* zero;
        Node* one;
    
        Node():zero(NULL), one(NULL){};
    };
    
    Node* root = new Node();
    
    int findMaximumXOR(vector<int>& nums) {
        int result = 0;
        if(nums.size() < 2) return result;
        if(nums.size() == 2) return nums[0]^nums[1];
        
        
        Node* cur_node;
        
        add_num(nums[0]);
        
        for(int i = 1; i < nums.size(); ++i){
            int cur = nums[i];
            int tem = compare_exist(cur);
            // cout << tem << endl;
            
            result = max(result, tem);
            add_num(cur);
        }
        
        return result;
    }
    
    int compare_exist(int num){
        Node* cur = root;
        int result = 0;
        
        for(int i = (1 << 30); i > 0; i >>= 1){
            if(num & i){
                if(cur->zero != NULL){
                    result |= i;
                    cur = cur->zero;
                }else{
                    cur = cur->one;
                }
            }else{
                if(cur->one != NULL){
                    result |= i;
                    cur = cur->one;
                }else{
                    cur = cur->zero;
                }
            }
        }
        
        return result;
    }
    
    void add_num(int num){
        Node* cur = root;
        
        for(int i = (1 << 30); i > 0; i >>= 1){
            if(num & i){
                if(cur->one == NULL) cur->one = new Node();
                cur = cur->one;
            }else{
                if(cur->zero == NULL) cur->zero = new Node();
                cur = cur->zero;
            }
        }
        
    }
};