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:
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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:
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Input: root = [1,0,1,0,0,0,1]
Output: [1,null,1,null,1]
Example 3:
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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 }
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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
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l := helper(root.Left)
r := helper(root.Right)
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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
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if !l && !r { return false } // 節點 value 不是 1,子節點也都沒1,回傳false,可剪
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return true
}
​
if !helper(root) {
return nil
}
​
return root
}
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