在二叉树中对具有特定值的节点进行计数

Question

I am trying to come up with a way to traverse a binary tree recursively versus iteratively and count how many times a find a specific value. One issue I'm running into is root in the first method.

Node inner class:

private class Node {

    int data;
    Node root;
    Node left;
    Node right;
}

Recursive & helper method:

public int valCount(int val) {
    if (root != null) {
        return valCount(val, root);
    }
    return 0;
}

public int valCount(int val, Node root) {
    int cnt = 0;
    if (root.left != null) {
        if (root.left.data == val) {
            cnt++;
        }
        valCount(val, root.left);
    }


    if (root.right != null) {
        if (root.right.data == val) {
            cnt++;
        }
        valCount(val, root.right);
    }
    return cnt;
}

I haven't been able to test because of the root issue so I'm not entirely sure my output will be correct. So, the question begs to be asked... am I even on the right track?? Does my approach even make sense? Any help would be awesome. Cheers!

Answer 1

Each time valCount is called, a separate copy of the local variable cnt is created. Thus when you call valCount for the root, this creates a variable cnt ; when valCount then calls itself for the left or right subtree, the new valCount has its own cnt , so when they increment cnt , they do not increment the cnt that the first valCount owns. This means that all the work done by valCount for the left and right subtrees is thrown away.

A simple way to fix this would be to notice that when you call valCount for the left or right subtree, the recursive call will return a value. You should use that value, instead of discarding the result:

int leftCount = valCount(val, root.left);

and then do something with leftCount (I'll let you think about how to use it).

One more thing: valCount should look at root.data , but it shouldn't look at root.left.data or root.right.data . Let the recursive calls do the work of looking at the data in the subtrees. That's how binary tree recursion often works.

Answer 2

You don't need "root" in your Node class. The "root" is the "this" object/pointer, right? Pass another parameter "int[] cnt" to valCount. Instantiate cnt as int[1]. Then in valCount do cnt[0]++. In the calling function after your recursion ends, print out cnt[0]. And remove the cnt local variable.

Answer 3

each time you call the method recursively a new object of cnt will be created that takes the value 0. Which means each time you increment it the value is lost. A good way to fix this is by placing cnt in the parameter of the method, and therefore each time you call it you pass the new value of cnt.