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# Dr. Evil Underscores

[1285D - Dr. Evil Underscores](https://codeforces.com/contest/1285/problem/D)

对于第`i`位，如果数组中所有的数都是`1`或者`0`，那么我们通过选择可以使异或的结果为`0`；

如果既有`1`也有`0`，异或的结果的`i`位一定是`1`，但是我们可以选择置`1`或者`0`的一个最小值。

```cpp
#include <iostream>
#include <vector>
using namespace std;

vector<int> a;
int solve(vector<int>& c, int bit) {
    if (c.size() == 0 || bit < 0) return 0;
    vector<int> l, r;
    for (auto& i : c) {
        if (((i >> bit) & 1) == 0) 
            l.push_back(i);
        else 
            r.push_back(i);
    }
    if (l.size() == 0) return solve(r, bit - 1);
    if (r.size() == 0) return solve(l, bit - 1);
    return min(solve(l, bit - 1), solve(r, bit - 1)) + (1 << bit);
}
int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int n;
    cin >> n;
    a.resize(n);
    for (auto& i : a) cin >> i;
    cout << solve(a, 30) << endl;
    return 0;
}
```