Introduction Link to heading

Given the root of a binary tree, each node in the tree has a distinct value.

After deleting all nodes with a value in to_delete, we are left with a forest (a disjoint union of trees).

Return the roots of the trees in the remaining forest. You may return the result in any order.

Example 1:

Input: root = [1,2,3,4,5,6,7], to_delete = [3,5]
Output: [[1,2,null,4],[6],[7]]

Constraints:

The number of nodes in the given tree is at most 1000.
Each node has a distinct value between 1 and 1000.
to_delete.length <= 1000
to_delete contains distinct values between 1 and 1000.

Solution Link to heading 

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func delNodes(root *TreeNode, to_delete []int) []*TreeNode {
    
    del := make([]bool, 1000)
    for _, val := range to_delete {
        del[val-1] = true
    }
    
    roots := make([]*TreeNode, 1000)
    roots[root.Val-1] = root
    
    q := []*TreeNode { root }
    for len(q) > 0 {
        
        node := q[0]
        q = q[1:]
        
        if del[node.Val-1] {
            roots[node.Val-1] = nil
            if node.Left != nil {
                roots[node.Left.Val-1] = node.Left
            }
            if node.Right != nil {
                roots[node.Right.Val-1] = node.Right
            }
        }
        
        if node.Left != nil {
            q = append(q, node.Left)
            if del[node.Left.Val-1] {
                node.Left = nil
            }
        }
        
        if node.Right != nil {
            q = append(q, node.Right)
            if del[node.Right.Val-1] {
                node.Right = nil
            }
        }        
    }

    out := []*TreeNode{}
    for _, v := range roots {
        if v != nil {
            out = append(out, v)
        }
    }
    
    return out
}

Explanation Link to heading

Let’s go by BFS through nodes and check deletes. Keep track of roots in special array (can be map). Return final values. Do not forget to delete actual nodes.