739. Daily Temperatures

Description

Given an array of integers temperatures represents the daily temperatures, return an array answer such that answer[i] is the number of days you have to wait after the ith day to get a warmer temperature. If there is no future day for which this is possible, keep answer[i] == 0 instead.

 

Example 1:

Input: temperatures = [73,74,75,71,69,72,76,73]
Output: [1,1,4,2,1,1,0,0]

Example 2:

Input: temperatures = [30,40,50,60]
Output: [1,1,1,0]

Example 3:

Input: temperatures = [30,60,90]
Output: [1,1,0]

 

Constraints:

• 1 <= temperatures.length <= 105
• 30 <= temperatures[i] <= 100

Solutions

Solution 1: Monotonic Stack

This problem requires us to find the position of the first element greater than each element to its right, which is a typical application scenario for a monotonic stack.

We traverse the array $\textit{temperatures}$ from right to left, maintaining a stack $\textit{stk}$ that is monotonically increasing from top to bottom in terms of temperature. The stack stores the indices of the array elements. For each element $\textit{temperatures}[i]$, we continuously compare it with the top element of the stack. If the temperature corresponding to the top element of the stack is less than or equal to $\textit{temperatures}[i]$, we pop the top element of the stack in a loop until the stack is empty or the temperature corresponding to the top element of the stack is greater than $\textit{temperatures}[i]$. At this point, the top element of the stack is the first element greater than $\textit{temperatures}[i]$ to its right, and the distance is $\textit{stk.top()} - i$. We update the answer array accordingly. Then we push $\textit{temperatures}[i]$ onto the stack and continue traversing.

After the traversal, we return the answer array.

The time complexity is $O(n)$, and the space complexity is $O(n)$. Here, $n$ is the length of the array $\textit{temperatures}$.

Python3JavaC++GoTypeScriptRustJavaScript

class Solution:
    def dailyTemperatures(self, temperatures: List[int]) -> List[int]:
        stk = []
        n = len(temperatures)
        ans = [0] * n
        for i in range(n - 1, -1, -1):
            while stk and temperatures[stk[-1]] <= temperatures[i]:
                stk.pop()
            if stk:
                ans[i] = stk[-1] - i
            stk.append(i)
        return ans

class Solution {
    public int[] dailyTemperatures(int[] temperatures) {
        int n = temperatures.length;
        Deque<Integer> stk = new ArrayDeque<>();
        int[] ans = new int[n];
        for (int i = n - 1; i >= 0; --i) {
            while (!stk.isEmpty() && temperatures[stk.peek()] <= temperatures[i]) {
                stk.pop();
            }
            if (!stk.isEmpty()) {
                ans[i] = stk.peek() - i;
            }
            stk.push(i);
        }
        return ans;
    }
}

class Solution {
public:
    vector<int> dailyTemperatures(vector<int>& temperatures) {
        int n = temperatures.size();
        stack<int> stk;
        vector<int> ans(n);
        for (int i = n - 1; ~i; --i) {
            while (!stk.empty() && temperatures[stk.top()] <= temperatures[i]) {
                stk.pop();
            }
            if (!stk.empty()) {
                ans[i] = stk.top() - i;
            }
            stk.push(i);
        }
        return ans;
    }
};

func dailyTemperatures(temperatures []int) []int {
    n := len(temperatures)
    ans := make([]int, n)
    stk := []int{}
    for i := n - 1; i >= 0; i-- {
        for len(stk) > 0 && temperatures[stk[len(stk)-1]] <= temperatures[i] {
            stk = stk[:len(stk)-1]
        }
        if len(stk) > 0 {
            ans[i] = stk[len(stk)-1] - i
        }
        stk = append(stk, i)
    }
    return ans
}

function dailyTemperatures(temperatures: number[]): number[] {
    const n = temperatures.length;
    const ans: number[] = Array(n).fill(0);
    const stk: number[] = [];
    for (let i = n - 1; ~i; --i) {
        while (stk.length && temperatures[stk.at(-1)!] <= temperatures[i]) {
            stk.pop();
        }
        if (stk.length) {
            ans[i] = stk.at(-1)! - i;
        }
        stk.push(i);
    }
    return ans;
}

impl Solution {
    pub fn daily_temperatures(temperatures: Vec<i32>) -> Vec<i32> {
        let n = temperatures.len();
        let mut stk: Vec<usize> = Vec::new();
        let mut ans = vec![0; n];

        for i in (0..n).rev() {
            while let Some(&top) = stk.last() {
                if temperatures[top] <= temperatures[i] {
                    stk.pop();
                } else {
                    break;
                }
            }
            if let Some(&top) = stk.last() {
                ans[i] = (top - i) as i32;
            }
            stk.push(i);
        }

        ans
    }
}

/**
 * @param {number[]} temperatures
 * @return {number[]}
 */
var dailyTemperatures = function (temperatures) {
    const n = temperatures.length;
    const ans = Array(n).fill(0);
    const stk = [];
    for (let i = n - 1; ~i; --i) {
        while (stk.length && temperatures[stk.at(-1)] <= temperatures[i]) {
            stk.pop();
        }
        if (stk.length) {
            ans[i] = stk.at(-1) - i;
        }
        stk.push(i);
    }
    return ans;
};

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