Cousins in Binary Tree II

At each level, first compute the sum of all original values of children on the next level. Then, for each parent at the current level, compute the sum of that parent's own children and assign every one of those children nextLevelTotal minus siblingSum. That value is exactly the sum of cousins: all next-level nodes excluding the child itself and its siblings. Set the root to zero. The key implementation detail is to calculate level totals and each parent’s sibling sum from values before overwriting child values; a two-phase level loop makes that explicit. A queue supports breadth-first traversal. Test a root-only tree, a parent with one child, and uneven levels. Each node is processed a constant number of times: O(n) time and O(width) queue space.