Amazon SDE - 1 interview questions

Two nodes in a binary search tree are swapped. We are supposed to find those two nodes, without any extra space (not even recursion) and O(N) TC. I was not able to come up with an approach without extra space. After around half hour of trying, he asked me to provide a solution which can use extra space. I gave it and he asked me to explain the whole working of the algorithm, step by step.