653. Two Sum IV - Input is a BST

Easy

Given the root of a Binary Search Tree and a target number k, return true if there exist two elements in the BST such that their sum is equal to the given target.

Example 1:

Input: root = [5,3,6,2,4,null,7], k = 9
Output:
 true

Example 2:

Input: root = [5,3,6,2,4,null,7], k = 28
Output:
 false

Constraints:

• The number of nodes in the tree is in the range [1, 104].

• -104 <= Node.val <= 104

• root is guaranteed to be a valid binary search tree.

• -105 <= k <= 105

解題

Runtime: 29 ms, faster than 81.78%

Memory Usage: 7.1 MB, less than 91.01%

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func findTarget(root *TreeNode, k int) bool {
    m := make(map[int]bool)
    
    var helper func(*TreeNode) bool
    helper = func(root *TreeNode) bool{
        if root == nil {
            return false
        }
    
        if m[k - root.Val] { return true }
        m[root.Val] = true
        
        return helper(root.Left) || helper(root.Right)
    }
    
    return helper(root)
}