617. Merge Two Binary Trees

class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        //If t1 == NULL -> t2
        //If t2 == NULL -> t2
        // Sum (t1,t2)
        
        //Preorder Traversal (P -> L -> R)
        if(t1 == null) return t2;
        if(t2 == null) return t1;
        t1.val+=t2.val;
        t1.left = mergeTrees(t1.left, t2.left);
        t1.right = mergeTrees(t1.right, t2.right);
        return t1;
    }
}



class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        //If t1 == NULL -> t2
        //If t2 == NULL -> t2
        // Sum (t1,t2)
        
        if(t1 == null) return t2;
        if(t2 == null) return t1;
        
        Stack<TreeNode[]> st = new Stack<>();
        st.push(new TreeNode[]{t1,t2});
        
        //While is not empty process the nodes
        while(!st.isEmpty()) {
            TreeNode[] curr = st.pop();
            
            //Process the node
           curr[0].val+=curr[1].val;
            
            //1. curr[0] = null, curr[1] != null -> curr[1]
            //2. curr[1] = null, curr[0] != null -> curr[0]
            //3. both not null, add it stack 
            
            //Left Tree
            if(curr[0].left == null) {
                curr[0].left = curr[1].left;
            }
            else if(curr[1].left != null) {
                st.push(new TreeNode[]{curr[0].left, curr[1].left});
            }
            
            //Right Tree
            if(curr[0].right == null) {
                curr[0].right = curr[1].right;
            }
            else if(curr[1].right != null) {
                st.push(new TreeNode[]{curr[0].right, curr[1].right});
            }
        }
        return t1;
    }
}


class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        //If t1 == NULL -> t2
        //If t2 == NULL -> t2
        // Sum (t1,t2)
        
        if(t1 == null) return t2;
        if(t2 == null) return t1;
        
        Queue<TreeNode[]> q = new LinkedList<>();
        q.add(new TreeNode[]{t1,t2});
        
        //While is not empty process the nodes
        while(!q.isEmpty()) {
            TreeNode[] curr = q.remove();
            if(curr[1] != null) {
                curr[0].val+=curr[1].val;
                
                //1. curr[0] = null -> curr[1]
                //2. curr[0] = null ->, add it queue 

                //Left Tree
                if(curr[0].left == null) {
                    curr[0].left = curr[1].left;
                }
                else {
                    q.add(new TreeNode[]{curr[0].left, curr[1].left});
                }

                //Right Tree
                if(curr[0].right == null) {
                    curr[0].right = curr[1].right;
                }
                else {
                    q.add(new TreeNode[]{curr[0].right, curr[1].right});
                }
            }
        }
        return t1;
    }
}