<p>Given an integer array <code>nums</code>, return <em>the maximum result of </em><code>nums[i] XOR nums[j]</code>, where <code>0 &lt;= i &lt;= j &lt; n</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [3,10,5,25,2,8]
<strong>Output:</strong> 28
<strong>Explanation:</strong> The maximum result is 5 XOR 25 = 28.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nums = [14,70,53,83,49,91,36,80,92,51,66,70]
<strong>Output:</strong> 127
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 2 * 10<sup>5</sup></code></li>
	<li><code>0 &lt;= nums[i] &lt;= 2<sup>31</sup> - 1</code></li>
</ul>

class Solution {
public:
    vector<vector<int>> bitmapArray;
    int GetMSB(int val) {
        int idx = 0;
        while(val > 1) {
            val >>= 1;
            idx++;
        }
        return idx;
    }
    bool BitOneIsPossible(const int targetVal, int bitpos, vector<int> &nums) {
        unordered_set<int> bitmap;
        for(auto n : nums) {
            n >>= bitpos;
            if(bitmap.find(targetVal ^ n) != bitmap.end())  return true;
            bitmap.insert(n);
        }
        return false;
    }
    int findMaximumXOR(vector<int>& nums) {
        const int msb = GetMSB(*max_element(nums.begin(),nums.end()));
        int largestXorVal = 0;
        for(int bitpos = msb ; bitpos >= 0 ; bitpos--) {
            largestXorVal <<= 1;
           if(BitOneIsPossible(largestXorVal | 1, bitpos, nums))   largestXorVal |= 1;
        }
        return largestXorVal;
    }
};

/*
record bitmap of each nums

the larger the msb, the larger the result after xor
find the largest element in nums ->largestnum
check its first zero-bit, check if any num's msb is this bit position

xor
1 ^ 1 = 0
1 ^ 0 = 1
0^ 0 = 0
largest。-> msb largest


check if possible:
    iterate through nums use unordered set to record each set. check if target ^ nums is in set 

convert all nums to bitmapArray
iterate from msb to lsb -> check if for that bit, 1 is possible
for each bit pos:
check if bitArray[0...pos] where pos == 1 is possible -> if yes push to bitArray
else push 0 to array  and move to next pos
*/
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