[Swift]LeetCode421. 數組中兩個數的最大異或值 | Maximum XOR of Two Numbers in an Array

原文地址：http://www.javashuo.com/article/p-mjbvkjpy-r.html 

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.

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給定一個非空數組，數組中元素爲 a0, a1, a2, … , an-1，其中 0 ≤ ai < 231 。

找到 ai 和aj 最大的異或 (XOR) 運算結果，其中0 ≤ i,  j < n 。

你能在O(n)的時間解決這個問題嗎？

示例:

輸入: [3, 10, 5, 25, 2, 8]

輸出: 28

解釋: 最大的結果是 5 ^ 25 = 28.

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196ms

 1 class Solution {
 2     func findMaximumXOR(_ nums: [Int]) -> Int {
 3         var res:Int = 0
 4         var mask:Int = 0
 5         for i in (0...31).reversed()
 6         {
 7             mask |= (1 << i)
 8             var s:Set<Int> = Set<Int>()
 9             for num in nums
10             {
11                 s.insert(num & mask)
12             }
13             var t:Int = res | (1 << i)
14             for prefix in s
15             {
16                 if s.contains(t ^ prefix)
17                 {
18                     res = t
19                     break
20                 }
21             }
22         }
23         return res
24     }
25 }

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872ms

 1 class TrieNode {
 2     var children = [Int: TrieNode]()
 3     var end: Bool
 4     var value: Int = -1
 5     
 6     init() {
 7         end = false
 8     }
 9     
10     func hasChild(_ key: Int) -> Bool {
11         return children[key] != nil
12     }
13     
14     func makeChild(_ key: Int) {
15         children[key] = TrieNode()
16     }
17     
18     func getChild(_ key: Int) -> TrieNode {
19         return children[key]!
20     }
21 }
22 
23 
24 class Solution {
25     
26     let INT_SIZE = 32
27     var root = TrieNode()
28     
29     func findMaximumXOR(_ nums: [Int]) -> Int {
30         
31         var ans = 0
32         insert(nums[0])
33         
34         for i in 1..<nums.count {
35             ans = max(ans, query(nums[i]))
36             insert(nums[i])
37         }
38         
39         return ans
40         
41     }
42     
43     func query(_ key: Int) -> Int {
44         var current = root 
45         
46         for i in (0..<INT_SIZE).reversed() {
47             let current_bit = (key & 1<<i) >= 1 ? 1 : 0
48             let next: TrieNode?
49             if current.hasChild(1-current_bit) {
50                 next = current.getChild(1-current_bit)
51             } else {
52                 next = current.getChild(current_bit)
53             } 
54             
55             current = next!
56         }
57         
58         return key ^ current.value
59         
60     }
61     
62     func insert(_ num: Int) {
63         
64         var current = root
65         
66         for i in (0..<INT_SIZE).reversed() {
67             let key = (num & 1<<i) >= 1 ? 1 : 0
68             if !current.hasChild(key) {
69                 current.makeChild(key)
70             }
71             
72             current = current.getChild(key)
73         }
74         
75         current.end = true
76         current.value = num
77     }
78 }