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public class Solution {
    
    TreeNode firstElement = null;
    TreeNode secondElement = null;
    // The reason for this initialization is to avoid null pointer exception in the first comparison when prevElement has not been initialized
    TreeNode prevElement = new TreeNode(Integer.MIN_VALUE);
    
    public void recoverTree(TreeNode root) {
        
        // In order traversal to find the two elements
        traverse(root);
        
        // Swap the values of the two nodes
        int temp = firstElement.val;
        firstElement.val = secondElement.val;
        secondElement.val = temp;
    }
    
    private void traverse(TreeNode root) {
        
        if (root == null)
            return;
            
        traverse(root.left);
        
        // Start of "do some business", 
        // If first element has not been found, assign it to prevElement (refer to 6 in the example above)
        if (firstElement == null && prevElement.val >= root.val) {
            firstElement = prevElement;
        }
    
        // If first element is found, assign the second element to the root (refer to 2 in the example above)
        if (firstElement != null && prevElement.val >= root.val) {
            secondElement = root;
        }        
        prevElement = root;

        // End of "do some business"

        traverse(root.right);
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void recoverTree(TreeNode root) {
        TreeNode first = null, second = null;
        TreeNode cur = root, pre = null;
        while (cur != null) {
            if (cur.left == null) {
                
                if (pre != null && pre.val > cur.val) {
                    if (first == null) {
                        first = pre;
                        second = cur;
                    } else {
                        second = cur;
                    }
                }
                pre = cur;
                cur = cur.right;
            } else {
                TreeNode temp = cur.left;
                while (temp.right != null && temp.right != cur) {
                    temp = temp.right;
                }
                if (temp.right == null) {
                    temp.right = cur;
                    //pre = cur;
                    cur = cur.left;
                } else {
                    
                    if (pre != null && pre.val > cur.val) {
                        if (first == null) {
                            first = pre;
                            second = cur;
                        } else {
                            second = cur;
                        }
                    }
                    temp.right = null;
                    pre = cur;
                    cur = cur.right;
                }
                
            } 
        }
        
        // swap two node values;
		if(first!= null && second != null){
		    int t = first.val;
		    first.val = second.val;
		    second.val = t;
		}
    }
}