Next greater or smaller element in array

stack_based.js

/*
Next greater element left or right
Next smaller element left or right
*/

// [1, 3, 2, 4] => [-1, 1, 1, 2]
// [1, 3, 0, 0, 1, 2, 4] => [-1, 1, -1, -1, 0, 1, 2]
function nextSmallerLeft(nums) {
  const ans = [];
  const stack = [];
  for (let i = 0; i < nums.length; i++) {
    while (stack.length && stack[0] >= nums[i]) {
      stack.shift();
    }
    ans[i] = stack.length ? stack[0] : -1;
    stack.unshift(nums[i]);
  }
  return ans;
}

// [1, 3, 2, 4] => [-1, 2, -1, -1]
// [1, 3, 0, 0, 1, 2, 4] => [0, 0, -1, -1, -1, -1, -1]
function nextSmallerRight(nums) {
  const ans = [];
  const stack = [];
  for (let i = nums.length - 1; i >= 0; i--) {
    while (stack.length && stack[0] >= nums[i]) {
      stack.shift();
    }
    ans[i] = stack.length ? stack[0] : -1;
    stack.unshift(nums[i]);
  }
  return ans;
}

// [1, 3, 2, 4] => [-1, -1, 3, -1]
// [1, 3, 0, 0, 1, 2, 4] => [-1, -1, 3, 3, 3, 3, -1]
function nextGreaterLeft(nums) {
  const ans = [];
  const stack = [];
  for (let i = 0; i < nums.length; i++) {
    while (stack.length && stack[0] <= nums[i]) {
      stack.shift();
    }
    ans[i] = stack.length ? stack[0] : -1;
    stack.unshift(nums[i]);
  }
  return ans;
}

// [1, 3, 2, 4] => [3, 4, 4, -1]
// [1, 3, 0, 0, 1, 2, 4] => [3, 4, 1, 1, 2, 4, -1]
function nextGreaterRight(nums) {
  const ans = [];
  const stack = [];
  for (let i = nums.length - 1; i >= 0; i--) {
    while (stack.length && stack[0] <= nums[i]) {
      stack.shift();
    }
    ans[i] = stack.length ? stack[0] : -1;
    stack.unshift(nums[i]);
  }
  return ans;
}