Consecutive Parent - Child

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
void dfs(TreeNode* A, long long par, int& res) {
    if(!A) return;
    res += abs(par - A->val) == 1;
    dfs(A->left, A->val, res);
    dfs(A->right, A->val, res);
}
int Solution::consecutiveNodes(TreeNode* A) {
    if(!A) return 0;
    int res = 0;
    dfs(A,A->val + 2ll,res);
    return res;
}