[email protected] [134] Gas station (Dynamic Programming)

https://leetcode.com/problems/gas-station/

There are N gas stations along a circular route, where the amount of gas at station i is gas[i].

You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from station i to its next station (i+1). You begin the journey with an empty tank at one of the gas stations.

Return the starting gas station‘s index if you can travel around the circuit once, otherwise return -1.

Note:
The solution is guaranteed to be unique.

class Solution {
public:
    int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
        int n = gas.size();
        if(n == 0) return -1;
        if(n == 1) {
            if(gas[0] - cost[0] >= 0)  return 0;
            return -1;
        }

        vector<int> dif; dif.clear();
        for(int i=0;i<n;++i) dif.push_back(gas[i] - cost[i]);
        for(int i=0;i<n-1;++i) dif.push_back(gas[i] - cost[i]);

        vector<int> sum(2*n-1, 0);
        vector<int> dp(2*n-1, 0);
        sum[0] = dif[0];
        dp[0] = 1;

        for(int i=1;i<2*n-1;++i) {
            if(sum[i-1] < 0) {
                sum[i] = dif[i];
                dp[i] = 1;
            }
            else {
                sum[i] = sum[i-1] + dif[i];
                dp[i] = dp[i-1] + 1;
            }

            if(dp[i] == n && sum[i] >= 0) return i-n+1;
        }

        return -1;
    }
};