653. Two Sum IV - Input is a BST

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653. Two Sum IV - Input is a BST

题目描述

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

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Example 1:
Input:
   5
  / \
 3   6
/ \   \
2   4   7
Target = 9
Output: True
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Example 2:
Input:
   5
  / \
 3   6
/ \   \
2   4   7
Target = 28
Output: False

Seen this question in a real interview before? YesNo

题目大意

在搜索二叉树内找到两个数和为target

解题思路

1、通过遍历二叉树，将二叉树转换为字典
2、遍历字典，寻找是否有两个数和为target

代码

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    func findTarget(_ root: TreeNode?, _ k: Int) -> Bool {
        var dict = [Int: Int]()
        func find(tree: TreeNode?) {
            guard let t = tree else { return }
            dict[t.val] = (dict[t.val] ?? 0) + 1
            find(tree: tree?.left)
            find(tree: tree?.right)
        }
        find(tree: root)
        for (key, _) in dict {
            if key == k - key  {
                if (dict[k - key] ?? 0) >= 2 {
                    return true
                }
            }else if dict[k - key] != nil {
                return true
            }
        }
        return false
    }
}
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