4 posts tagged with "stack"

/**
 * 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 {number[]}
 */
var preorderTraversal = function (root) {
  let result = [];

  traversalHelper(root, result);

  return result;
};

const traversalHelper = (root, result) => {
  if (root === null) {
    return;
  }
  result.push(root.val);
  traversalHelper(root.left, result);
  traversalHelper(root.right, result);
};

/**
 * 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 {number[]}
 */
var postorderTraversal = function (root) {
  let result = [];
  traversalHelper(root, result);
  return result;
};

const traversalHelper = (root, result) => {
  if (root === null) {
    return;
  }
  result.unshift(root.val);
  traversalHelper(root.right, result);
  traversalHelper(root.left, result);
};

/**
 * @param {string} s
 * @return {boolean}
 */
var isValid = function (s) {
  if (s.length % 2 !== 0) {
    return false;
  }

  const stack = [];
  const brackets = {
    '(': ')',
    '{': '}',
    '[': ']',
  };

  for (let i = 0; i < s.length; i++) {
    if (Object.keys(brackets).includes(s[i])) {
      stack.push(s[i]);
    } else {
      if (brackets[stack[stack.length - 1]] === s[i]) {
        stack.pop();
      } else {
        return false;
      }
    }
  }
  return stack.length === 0;
};

/**
 * 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 {number[]}
 */
var inorderTraversal = function (root) {
  let stack = [];
  let result = [];

  while (root !== null || stack.length !== 0) {
    while (root !== null) {
      stack.push(root);
      root = root.left;
    }
    root = stack.pop();
    result.push(root.val);
    root = root.right;
  }
  return result;
};