1325.RemoveLeafNodes.cs

// 1325. 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 its parent node becomes a leaf node and has the value target, it should also be deleted (you need to continue doing that until you cannot).

// 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.

// Constraints:
// The number of nodes in the tree is in the range [1, 3000].
// 1 <= Node.val, target <= 1000
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {
    public TreeNode RemoveLeafNodes(TreeNode root, int target) {
        if(TargetLeafNodes(root, target))
                return null;
        return root;
    }
    private bool TargetLeafNodes(TreeNode root, int target) {
        if(root == null)
            return true;
        if(TargetLeafNodes(root.left, target))
            root.left = null;
        if(TargetLeafNodes(root.right, target))
            root.right = null;
        if(root.right == null && root.left == null && root.val == target){
            return true;
        }
        return false;
    }
}