Software Engineer III

Tip 1 : Practice multithreading concepts with examples
Tip 2 : Ready garbage collection process and memory management in java
Tip 3 : Read about different collections and their different use case, this would really help in answering the questions

Method 1 (Count 0s or 1s) :
Step1: count zeros and ones
Step2 : put zeros first in array and then 1s in array

Method 2:

Method 2 (Use two indexes to traverse) 
Maintain two indexes. Initialize the first index left as 0 and second index right as n-1.
Do following while left < right 
a) Keep incrementing index left while there are 0s at it 
b) Keep decrementing index right while there are 1s at it 
c) If left < right then exchange arr[left] and arr[right]

public List> zigzagLevelOrder(TreeNode root) {

List> ans = new ArrayList();
if(root == null ) return ans;

Deque queue = new ArrayDeque();
int count = -1;
queue.add(root);
while(!queue.isEmpty()){
List list = new ArrayList();
int n = queue.size();
if(count == -1 ){
for(int i=0; i TreeNode node = queue.removeFirst();
list.add(node.val);
if(node.left != null ) queue.addLast(node.left);
if(node.right != null ) queue.addLast(node.right);
}
}else{
for(int i=0; i TreeNode node = queue.removeLast();
list.add(node.val);

if(node.right != null ) queue.addFirst(node.right);
if(node.left != null ) queue.addFirst(node.left);
}

}
ans.add(list);
count = -count;
}
return ans;
}