Mystery null pointer exception error in code ?

class AVLTree_INC {

Node root;

int height(Node N) {

if (N == null)

return 0;

return N.height;

}

int max(int a, int b) {

return (a > b) ? a : b;

}

Node rightRotate(Node y) {

Node x = y.left;

Node T2 = x.right;

x.right = y;

y.left = T2;

y.height = max(height(y.left), height(y.right)) + 1;

x.height = max(height(x.left), height(x.right)) + 1;

return x;

}

Node leftRotate(Node x) {

Node y = x.right;

Node T2 = y.left;

y.left = x;

x.right = T2;

x.height = max(height(x.left), height(x.right)) + 1;

y.height = max(height(y.left), height(y.right)) + 1;

return y;

}

int getBalance(Node N) {

if (N == null)

return 0;

return height(N.left) - height(N.right);

}

Node insert(Node node, int key) {

if (node == null)

return (new Node(key));

if (key

node.left = insert(node.left, key);

else if (key > node.key)

node.right = insert(node.right, key);

else

return node;

node.height = 1 + max(height(node.left),

height(node.right));

int balance = getBalance(node);

if (balance > 1 && key

return rightRotate(node);

if (balance node.right.key)

return leftRotate(node);

if (balance > 1 && key > node.left.key) {

node.left = leftRotate(node.left);

return rightRotate(node);

}

if (balance

node.right = rightRotate(node.right);

return leftRotate(node);

}

return node;

}

public Node search(Node root, int key) {

if (root==null || root.key==key)

return root;

if (root.key > key)

return search(root.left, key);

return search(root.right, key);

}

void builder(AVLTree_INC tree, int a[]){

int c=0;

for(int i=1;i

long begin = System.nanoTime();

System.out.println("Batch " + i);

for(int j=0;j

tree.root = tree.insert(tree.root, a[c++]);

}

long end = System.nanoTime();

long elapsed_time = end - begin;

System.out.println(elapsed_time);

}

}

Kterminated Homework five 124 [Java Application] /Library/Java/JavavirtualMachinesljdk1.8.0 71.j Batch 10 4605522 AVL Tree RAND. txt Batch 1 Exception in thread "main" java Lang.Null PointerException at homework.AVLTree INC. LeftRotate CHomework five 124 java: 135 at homework.AVLTree INC. insert Homework five 124. java: 179) at homework .AVLTree INC. insertCHomework five 124. java: 163) at homework. AvLTree INc.insert Homework five 124. java: 163 at homework .AVLTree INC. insert Homework five 124. java: 161) at homework.AVLTree INC. insert Homework five 124. java: 163) at homework .AVLTree INC. insertCHomework five 124. java: 163) at homework. AvLTree INc.insert Homework five 124. java: 161) at homework .AVLTree INC. insertCHomework five 124. java: 161) at homework.AVLTree-INC. insertCHomework five 124 java: 161) at homework .AVLTree INC. insertCHomework five 124. java: 161) at homework.AVLTree INC. insert Homework five 124 java: 161) at homework. AVLTree INC. builder CHomework five 124 java 208) at homework. Homework-five 124.mainCHomework five 124 java: 517 Kterminated Homework five 124 [Java Application] /Library/Java/JavavirtualMachinesljdk1.8.0 71.j Batch 10 4605522 AVL Tree RAND. txt Batch 1 Exception in thread "main" java Lang.Null PointerException at homework.AVLTree INC. LeftRotate CHomework five 124 java: 135 at homework.AVLTree INC. insert Homework five 124. java: 179) at homework .AVLTree INC. insertCHomework five 124. java: 163) at homework. AvLTree INc.insert Homework five 124. java: 163 at homework .AVLTree INC. insert Homework five 124. java: 161) at homework.AVLTree INC. insert Homework five 124. java: 163) at homework .AVLTree INC. insertCHomework five 124. java: 163) at homework. AvLTree INc.insert Homework five 124. java: 161) at homework .AVLTree INC. insertCHomework five 124. java: 161) at homework.AVLTree-INC. insertCHomework five 124 java: 161) at homework .AVLTree INC. insertCHomework five 124. java: 161) at homework.AVLTree INC. insert Homework five 124 java: 161) at homework. AVLTree INC. builder CHomework five 124 java 208) at homework. Homework-five 124.mainCHomework five 124 java: 517