DAFUQ

nodes[curIndex].left = build(from, from + psize, depth + 1);
nodes[curIndex].right = build(from + psize + 1, to, depth + 1);
//^ causes segfault

int lf = build(from, from + psize, depth + 1);
nodes[curIndex].left = lf;
int rf = build(from + psize + 1, to, depth + 1);
nodes[curIndex].right = rf;
//^ works fine