The major trick is from another similar HR problem: subarray with max XOR:
Build a binary tree bit by bit, and go from MSB to LSB, greedily.
In this one, we need one more step before using that trick. We need do "rationally reason" and visualize it: we separate ints into 2 groups as editorial mentioned - one with MSB-diff on, the other with MSB-diff off. And use the trick above to find min one.