Solution.ts

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function longestConsecutive(root: TreeNode | null): number {
    let ans = 0;
    const dfs = (root: TreeNode | null): number => {
        if (root === null) {
            return 0;
        }
        let l = dfs(root.left) + 1;
        let r = dfs(root.right) + 1;
        if (root.left && root.left.val - root.val !== 1) {
            l = 1;
        }
        if (root.right && root.right.val - root.val !== 1) {
            r = 1;
        }
        const t = Math.max(l, r);
        ans = Math.max(ans, t);
        return t;
    };
    dfs(root);
    return ans;
}