Bifalgorithm post

"Three techniques you already know, stacked into one problem. Take one to five and reorder it. First last, second, second last, and so on. In place, no extra array. Leak code 143, reorder list. Here's why this isn't just a loop. You need the last node, then the second last, then the third last. But a linked list only points forward. Walking back to the end, every single time is order n squared. And copying it into an array is the answer nobody wants to hear. Phase 1. Find the middle. Slow and fast pointers, straight from chapter 5. Slow moves one step, fast moves two. When fast runs out of room, slow is sitting on the end of the first half. Cut there and you have 1-2-3 and 4-5. Phase 2. Reverse the back half. That's reverse a link list, applied to 4-5. Save, flip, advance, advance, and 4-5 becomes 5-4. Now the back half runs backwards, so its head is the last node of the original list. Phase 3. Weave them. One node from the front, one from the reversed back, alternating. 1-5-2-4-3. Save both next pointers before you relink, or you lose the rest. Same rule as always. And that's the answer in place. All three phases, one function. Find the middle with slow and fast. Cut and reverse what comes after the cut. Then walk both halves at once, relinking as you go. Order end time, order one space. And notice, there is not one new idea in here. It's chapter 5, plus reverse a linked list, plus a weave. This is why reversing a list mattered. Reorder list is a combo, and reversal is the move it leans on hardest. Next up, the last one in this chapter, and the one FAA&G loves most. LRU Cache, where linked lists meet system design. Save this before your next interview, and don't forget to like. Follow so you don't miss it."

"Three techniques you already know, stacked into one problem. Take one to five and reorder it. First last, second, second last, and so on. In place, no extra a…