814. Binary Tree Pruning

Medium

Given the root of a binary tree, return the same tree where every subtree (of the given tree) not containing a 1 has been removed.

A subtree of a node node is node plus every node that is a descendant of node.

Example 1:

Input: root = [1,null,0,0,1]
Output: [1,null,0,null,1]
Explanation: 
Only the red nodes satisfy the property "every subtree not containing a 1".
The diagram on the right represents the answer.

Example 2:

Input: root = [1,0,1,0,0,0,1]
Output: [1,null,1,null,1]

Example 3:

Input: root = [1,1,0,1,1,0,1,0]
Output: [1,1,0,1,1,null,1]

Constraints:

• The number of nodes in the tree is in the range [1, 200].

• Node.val is either 0 or 1.

解題

Runtime: 0 ms, faster than 100%

Memory Usage: 2.2 MB, less than 68.42%

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func pruneTree(root *TreeNode) *TreeNode {
    if root == nil { return nil }

    var helper func(*TreeNode) bool
    helper = func(root *TreeNode) bool {
        if root == nil { return false } // nil 必定不包含 1，回傳 false

        l := helper(root.Left) 
        r := helper(root.Right) 

        if !l { root.Left = nil } // 沒1，修剪左邊子節點
        if !r { root.Right = nil } // 沒1，修剪右邊子節點
        
        if root.Val == 1 { return true } // 節點 value 是 1，回傳true，不要剪掉

        if !l && !r { return false } // 節點 value 不是 1，子節點也都沒1，回傳false，可剪

        return true
    }

    if !helper(root) { 
        return nil
    }

    return root
}