Solution.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {void} Do not return anything, modify root in-place instead.
 */
var recoverTree = function (root) {
    let prev = null;
    let first = null;
    let second = null;
    function dfs(root) {
        if (!root) {
            return;
        }
        dfs(root.left);
        if (prev && prev.val > root.val) {
            if (!first) {
                first = prev;
            }
            second = root;
        }
        prev = root;
        dfs(root.right);
    }
    dfs(root);
    const t = first.val;
    first.val = second.val;
    second.val = t;
};