31. Delete Leaves With a Given Value

Given a binary tree root and an integer target, delete all the leaf nodes with value target.

Note that once you delete a leaf node with value target, if it's parent node becomes a leaf node and has the value target, it should also be deleted (you need to continue doing that until you can't).

Example 1:

Input: root = [1,2,3,2,null,2,4], target = 2
Output: [1,null,3,null,4]
Explanation: Leaf nodes in green with value (target = 2) are removed (Picture in left). 
After removing, new nodes become leaf nodes with value (target = 2) (Picture in center).

Example 2:

Input: root = [1,3,3,3,2], target = 3
Output: [1,3,null,null,2]

Example 3:

Input: root = [1,2,null,2,null,2], target = 2
Output: [1]
Explanation: Leaf nodes in green with value (target = 2) are removed at each step.

Example 4:

Input: root = [1,1,1], target = 1
Output: []

Example 5:

Input: root = [1,2,3], target = 1
Output: [1,2,3]

Solution: (Postorder Traversal)

class Solution
{
public:
    void remove(TreeNode *&root, int &target)
    {
        if (root == NULL)
        {
            return;
        }

        remove(root->left, target);
        remove(root->right, target);

        if (root->left == NULL && root->right == NULL)
        {
            if (root->val == target)
            {
                root = NULL;
                delete root;
            }
            return;
        }

        return;
    }

    TreeNode *removeLeafNodes(TreeNode *root, int target)
    {

        remove(root, target);
        return root;
    }
};