https://leetcode-cn.com/problems/sum-root-to-leaf-numbers/

    1. /**
    2.  * Definition for a binary tree node.
    3.  * class TreeNode {
    4.  *     val: number
    5.  *     left: TreeNode | null
    6.  *     right: TreeNode | null
    7.  *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
    8.  *         this.val = (val===undefined ? 0 : val)
    9.  *         this.left = (left===undefined ? null : left)
    10.  *         this.right = (right===undefined ? null : right)
    11.  *     }
    12.  * }
    13.  */
    15. function sumNumbers(root: TreeNode | null): number {
    16.   return dfs(root, 0);
    17. }
    19. function dfs(root: TreeNode | null, num: number) {
    20.   if (root === null) return 0;
    21.   const temp = num * 10 + root.val;
    22.   if (root.left === null && root.right === null) {
    23.     return temp;
    24.   }
    25.   return dfs(root.left, temp) + dfs(root.right, temp);
    26. }

    https://bigfrontend.dev/zh/problem/remove-characters

    /**
 * @param {string} input
 * @returns string
 */
function removeChars(input) {
  const stack = [];
  for (let i = 0; i < input.length; i++) {
    const char = input[i];
    if (stack.length && char === 'c' && stack[stack.length - 1] === 'a') {
      stack.pop();
    } else if (char !== 'b') {
      stack.push(char);
    }
  }
  return stack.join('');
}