How to determine whether a binary tree is complete?

Question

A complete binary tree is defined as a binary tree in which every level, except possibly the deepest, is completely filled. At deepest level, all nodes must be as far left a

Answer 1

You can also solve this problem by using level order traversal. The procedure is as follows:

1. Store the data element of the nodes encountered in a vector
2. If the node is NULL, then store a special number(INT_MIN)
3. Keep track of the last non-null node visited - lastentry
4. Now traverse the vector till lastentry. If you ever encounter INT_MIN, then the tree is not complete. Else it is a complete binary tree.

Here is a c++ code:

My tree node is:

struct node{
    int data;
    node *left, *right;
};

void checkcomplete(){//checks whether a tree is complete or not by performing level order traversal
    node *curr = root;
    queue Q;
    vector arr;
    int lastentry = 0;
    Q.push(curr);
    int currlevel = 1, nextlevel = 0;
    while( currlevel){
        node *temp = Q.front();
        Q.pop();
        currlevel--;
        if(temp){
            arr.push_back(temp->data);
            lastentry = arr.size();
            Q.push(temp->left);
            Q.push(temp->right);
            nextlevel += 2;
        }else
            arr.push_back(INT_MIN);
        if(!currlevel){
            currlevel = nextlevel;
            nextlevel = 0;
        }
    }
    int flag = 0;
    for( int i = 0; i