739.Daily-Temperatures

739. Daily Temperatures

题目地址

https://leetcode.com/problems/daily-temperatures/

题目描述

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].

代码

Approach #1 Stack

从右往左➡️，构建单调递减栈⏬

Time Complexity: O(N) && Space Complexity: O(W)

class Solution {
  public int[] dailyTemperatures(int[] T) {
    int[] ans = new int[T.length];
    Stack<Integer> stack = new Stack();
    for (int i = T.length - 1; i >= 0; i--) {
      while (!stack.isEmpty() && T[i] >= T[stack.peek()]) {
        stack.pop();
      }

      if (stack.isEmpty()) {
        ans[i] = 0;
      } else {
        ans[i] = stack.peek() - i;
      }

      stack.push(i);
    }
    return ans;
  }
}

Approach 2: Next Array

Time Complexity: O(NW) && Space Complexity: O(N + W)

class Solution {
   public int[] dailyTemperatures(int[] T) {
    int[] ans = new int[T.length];
    int[] next = new int[101];
    Arrays.fill(next, Integer.MAX_VALUE);
    for (int i = T.length - 1; i >= 0; i--) {
      int warmer_index = Integer.MAX_VALUE;
      for (int t = T[i] + 1; t <= 100; t++) {
        if (next[t] < warmer_index) {
          warmer_index = next[t];
        }
      }
      if (warmer_index < Integer.MAX_VALUE) {
        ans[i] = warmer_index - 1; // -1 ?
      }
      next[T[i]] = i;
    }

    return ans;
   }
}