653. Two Sum IV - Input is a BST

最新推荐文章于 2020-05-09 10:47:16 发布

转载最新推荐文章于 2020-05-09 10:47:16 发布 · 220 阅读

本文介绍了一种在二叉搜索树中查找是否存在两个元素之和等于给定目标值的方法。通过三种不同的实现方式：中序遍历后使用双指针技巧、使用两个迭代器分别从树的两端遍历以及递归深度优先搜索配合哈希集合检查，有效地解决了这一问题。

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.

Example 1:

Input: 
    5
   / \
  3   6
 / \   \
2   4   7

Target = 9

Output: True

Example 2:

Input: 
    5
   / \
  3   6
 / \   \
2   4   7

Target = 28

Output: False

class Solution {
public:
    bool findTarget(TreeNode* root, int k) {
        vector<int> nums;
        inorder(root, nums);
        return findTargetInSortedArray(nums, k);
    }

private:
    void inorder(TreeNode* node, vector<int>& nums) {
        if (!node) return;
        inorder(node->left, nums);
        nums.push_back(node->val);
        inorder(node->right, nums);
    }

    bool findTargetInSortedArray(vector<int> a, int target) {
        for (int i = 0, j = a.size() - 1; i < j;) {
            int sum = a[i] + a[j];
            if (sum == target) {
                return true;
            }
            else if (sum < target) {
                i++;
            }
            else {
                j--;
            }
        }

        return false;
    }
};

class BSTIterator {
    stack<TreeNode*> s;
    TreeNode* node;
    bool forward;
public:
    BSTIterator(TreeNode *root, bool forward) : node(root), forward(forward) {};
    bool hasNext() {
        return node != nullptr || !s.empty();
    }
    int next() {
        while (node || !s.empty()) {
            if (node) {
                s.push(node);
                node = forward ? node->left : node->right;
            }
            else {
                node = s.top();
                s.pop();
                int nextVal = node->val;
                node = forward ? node->right : node->left;
                return nextVal;
            }
        }

        return -1;  // never reach & not necessary
    }
};
class Solution {
public:
    bool findTarget(TreeNode* root, int k) {
        if (!root) return false;
        BSTIterator l(root, true);
        BSTIterator r(root, false);
        for (int i = l.next(), j = r.next(); i < j;) {
            int sum = i + j;
            if (sum == k) {
                return true;
            }
            else if (sum < k) {
                i = l.next();
            }
            else {
                j = r.next();
            }
        }
        return false;
    }
};

 bool findTarget(TreeNode* root, int k) {
        unordered_set<int> set;
        return dfs(root, set, k);
    }
    
    bool dfs(TreeNode* root, unordered_set<int>& set, int k){
        if(root == NULL)return false;
        if(set.count(k - root->val))return true;
        set.insert(root->val);
        return dfs(root->left, set, k) || dfs(root->right, set, k);
    }