LC 962. Maximum Width Ramp

Given an array A of integers, a ramp is a tuple (i, j) for which i < j and A[i] <= A[j].  The width of such a ramp is j - i.

Find the maximum width of a ramp in A.  If one doesn't exist, return 0.

 

Example 1:

Input: [6,0,8,2,1,5]
Output: 4
Explanation: 
The maximum width ramp is achieved at (i, j) = (1, 5): A[1] = 0 and A[5] = 5.

Example 2:

Input: [9,8,1,0,1,9,4,0,4,1]
Output: 7
Explanation: 
The maximum width ramp is achieved at (i, j) = (2, 9): A[2] = 1 and A[9] = 1.

 

Note:

1. 2 <= A.length <= 50000
2. 0 <= A[i] <= 50000

直接的做法，O(nlog(n))。

class Solution {
public:
    int maxWidthRamp(vector<int>& A) {
        map<int, vector<int>> m;
        for(int i=0; i<A.size(); i++) m[A[i]].push_back(i);
        int ret = -1;
        int prev_first = 0;
        for(auto it : m){
            //cout << it.first << endl;
            if(ret == -1){
                prev_first = it.second.front();
                ret = max(ret, it.second.back() - prev_first);
            }else{
                prev_first = min(prev_first, it.second.front());
                ret = max(ret, it.second.back() - prev_first);
            }
        }
        return ret;
    }
};