需要关注一下单调栈：单调栈用途不太广泛，只处理一类典型的问题，比如「下一个更大元素」，「上一个更小元素」等。
https://leetcode.com/problems/daily-temperatures/
https://leetcode.com/problems/shortest-unsorted-continuous-subarray/
https://leetcode.com/problems/next-greater-element-i/
https://leetcode.com/problems/next-greater-element-ii/
https://leetcode.com/problems/online-stock-span/
https://leetcode.com/problems/132-pattern/
https://leetcode.com/problems/sum-of-subarray-minimums/
https://leetcode.com/problems/largest-rectangle-in-histogram/