LeetCode 99: Recover Binary Search Tree

Difficulty: 4

Frequency: 2

Problem:

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.
Note:
A solution using O(n)
space is pretty straight forward. Could you devise a constant space solution?

Solution:

class Solution {
public:
void recoverTree(TreeNode *root) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
TreeNode *p_last, *p_small, *p_big;
bool b_big = true;
p_small = p_big = NULL;
int i_last = INT_MIN;
Traverse(root, i_last, p_last, p_small, p_big, b_big);
i_last = p_small->val;
p_small->val = p_big->val;
p_big->val = i_last;
}
void Traverse(TreeNode * root, int & i_last, TreeNode * & p_last, TreeNode *& p_small, TreeNode *& p_big, bool & b_big)
{
if (root==NULL)
return;

if (root->left)
Traverse(root->left, i_last, p_last, p_small, p_big, b_big);

if (i_last>root->val)
{
if (b_big)
{
p_big = p_last;
b_big = !b_big;
p_small = root;// Doing this is because the swapped two may be adjacent.
}
else
{
p_small = root;
return;
}
}
i_last = root->val;
p_last = root;
if (root->right)
Traverse(root->right, i_last, p_last, p_small, p_big, b_big);
}
};

Notes:

Lots of subtleties.