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

Here is a C code for checking if the binary tree is complete:

struct node
{
    int data;
    struct node * left;
    struct node * right;
};
int flag;
int isComplete(struct node *root, int depth)
{
    int ld, rd;
    if (root==NULL) return depth;
    else
    {
        ld =  isComplete(root->left,depth+1);
        rd = isComplete(root->right, depth+1);
        if (ld==-1 || rd==-1) return -1;
        else if (ld==rd) return ld;
        else if (ld==rd-1 && flag==0)
        {
            flag=1;
            return rd;
        }
        else return -1;
    }
}

The way it works is:

• If the depth of left subtree is same as depth of right subtree, it returns the depth up the hirearchy.

• if the depth of left subtree is one more than the depth of right subtree, it returns depth of right subtree up the hirarchy and enables the flag.

• If it finds that the depth of left subtree and right subtree and flag is already set, it returns -1 up the hierarchy.

• In the end, if the function returns -1, it is not the complete subtree, else the value returned is the minimum depth of the tree.