141 2-sum-in-BST.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void findInorder(TreeNode* root, vector<int> &inorder) {
        if(root==NULL)
            return;
        
        findInorder(root->left,inorder);
        inorder.push_back(root->val);
        findInorder(root->right,inorder);
    }

    bool findTarget(TreeNode* root, int k) {
        // Approach 1: By find inorder
        // TC: O(N)
        // SC: O(N)
        
        vector<int> inorder;
        findInorder(root,inorder);
        
        int i=0,j=inorder.size()-1;
        while(i<j) {
            if(inorder[i]+inorder[j]==k) {
                return true;
            }
            else if(inorder[i]+inorder[j]<k) {
                i++;
            }
            else {
                j--;
            }
        }
        
        return false;
        
        // Approach 2: Using BST iterator before() and next() operations and applying 2-pointer approach
        // TC: O(N)
        // SC: O(2H)
    }
};