CSES 1668 Building Teams - Walkthrough

let's trace $n = 4$, $m = 3$, edges: $(1,2)$, $(2,3)$, $(3,4)$. This is a path: $1$-$2$-$3$-$4$. BFS from $1$: color[1] = 0. Neighbor $2$: color[2] = 1. Neighbor of $2$: $1$ (already $0$, good), $3$ (color[3] = 0).

Neighbor of $3$: $2$ (already $1$, good), $4$ (color[4] = 1). Final: color = [-1, 0, 1, 0, 1]. Output: "$1$ $2$ $1$ $2$" (adding $1$ to each). Team $1$: pupils $1$, $3$. Team $2$: pupils $2$, $4$.