> For the complete documentation index, see [llms.txt](https://junnie.gitbook.io/nine-chapter/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://junnie.gitbook.io/nine-chapter/1.binary-search/585maximum-number-in-mountain-sequence.md).

# 585.Maximum Number in Mountain Sequence

## 1.Description(Medium)

Given a mountain sequence of`n`integers which increase firstly and then decrease, find the mountain top.

**Example**

Given`nums`=`[1, 2, 4, 8, 6, 3]`return`8`\
Given`nums`=`[10, 9, 8, 7]`, return`10`

## 2.Code

这道题用的是三分，而不是二分\
三分是用两个mid的\
二分处理的是，连续上升或连续下降的问题\
三分处理的是，连续上升接连续下降的问题

二分处理的是有序的问题，如递增，或者递减

三分解决的是极值问题，也就是凸函数，如 y = x^2, 求最小值，你可以在\[-oo, oo]区间内三分

```
public int mountainSequence(int[] nums) {
        if(nums==null || nums.length==0){
            return -1;
        }
        int left=0;
        int right=nums.length-1;
        while(left+1<right){
            int mid1=left+(right-left)/2;
            int mid2=right-(right-mid1)/2;
            if(nums[mid1]<nums[mid2]){
                left=mid1+1;
            }else if(nums[mid1]>nums[mid2]){
                right=mid2-1;
            }else{
                left=mid1;
                right=mid2;
            }
        }
        return nums[left]>nums[right]?nums[left]:nums[right];

    }
```