Functions abs is available if you include <stdlib.h>. (abs(x) is the absolute value of x.)
int max(int x, int y) { if(x > y) return x; else return y; } int heightBalanced(const Node* t) { if(t == NULL) return 0; else { int x = heightBalanced(t->left); int y = heightBalanced(t->right); if(x >= 0 && abs(x - y) <= 1) return max(x,y) + 1; else return -1; } }