739. Daily Temperatures #

Problem #

Given a list of daily temperatures T, return a list such that, for each day in the input, tells you how many days you would have to wait until a warmer temperature. If there is no future day for which this is possible, put 0 instead.

For example, given the list of temperatures T = [73, 74, 75, 71, 69, 72, 76, 73], your output should be [1, 1, 4, 2, 1, 1, 0, 0].

Note: The length of temperatures will be in the range [1, 30000]. Each temperature will be an integer in the range [30, 100].

Problem Summary #

Given an array of temperatures, output how many days in the future a higher temperature than the current day’s temperature will occur. For example, a temperature higher than 73 degrees appears on the 1st future day, and a temperature higher than 75 degrees appears on the 4th future day.

Solution Approach #

This problem can be handled normally according to the problem statement, using two nested loops. Another approach is to use a monotonic stack; just maintain a monotonically decreasing stack.

Code #

package leetcode

// Solution 1: brute-force approach
func dailyTemperatures(T []int) []int {
	res, j := make([]int, len(T)), 0
	for i := 0; i < len(T); i++ {
		for j = i + 1; j < len(T); j++ {
			if T[j] > T[i] {
				res[i] = j - i
				break
			}
		}
	}
	return res
}

// Solution 2: monotonic stack
func dailyTemperatures1(T []int) []int {
	res := make([]int, len(T))
	var toCheck []int
	for i, t := range T {
		for len(toCheck) > 0 && T[toCheck[len(toCheck)-1]] < t {
			idx := toCheck[len(toCheck)-1]
			res[idx] = i - idx
			toCheck = toCheck[:len(toCheck)-1]
		}
		toCheck = append(toCheck, i)
	}
	return res
}