Leetcode 75

Solution/739. Daily Temperatures/739. Daily Temperatures.py

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+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

Solution/739. Daily Temperatures/readme.md

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+---
+comments: true
+difficulty: Medium
+edit_url: Antim
+tags:
+    - Stack
+    - Array
+    - Monotonic Stack
+---
+
+<!-- problem:start -->
+
+# [739. Daily Temperatures](https://leetcode.com/problems/daily-temperatures)
+
+## Description
+
+<!-- description:start -->
+
+<p>Given an array of integers <code>temperatures</code> represents the daily temperatures, return <em>an array</em> <code>answer</code> <em>such that</em> <code>answer[i]</code> <em>is the number of days you have to wait after the</em> <code>i<sup>th</sup></code> <em>day to get a warmer temperature</em>. If there is no future day for which this is possible, keep <code>answer[i] == 0</code> instead.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<pre><strong>Input:</strong> temperatures = [73,74,75,71,69,72,76,73]
+<strong>Output:</strong> [1,1,4,2,1,1,0,0]
+</pre><p><strong class="example">Example 2:</strong></p>
+<pre><strong>Input:</strong> temperatures = [30,40,50,60]
+<strong>Output:</strong> [1,1,1,0]
+</pre><p><strong class="example">Example 3:</strong></p>
+<pre><strong>Input:</strong> temperatures = [30,60,90]
+<strong>Output:</strong> [1,1,0]
+</pre>
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;=&nbsp;temperatures.length &lt;= 10<sup>5</sup></code></li>
+	<li><code>30 &lt;=&nbsp;temperatures[i] &lt;= 100</code></li>
+</ul>
+
+<!-- description:end -->
+
+## Solutions
+
+<!-- solution:start -->
+
+### 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 $\textit{temperatures}$.
+
+<!-- tabs:start -->
+
+#### Python3
+
+```python
+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
+```
+
+#### Java
+
+```java
+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;
+    }
+}
+```
+
+#### C++
+
+```cpp
+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;
+    }
+};
+```
+
+<!-- tabs:end -->
+
+<!-- solution:end -->
+
+<!-- problem:end -->