45. Jump Game II

Given an array of non-negative integers, you are initially positioned at the first index of the array.

Each element in the array represents your maximum jump length at that position.

Your goal is to reach the last index in the minimum number of jumps.

Example:

Input: [2,3,1,1,4]
Output: 2
Explanation: The minimum number of jumps to reach the last index is 2.
    Jump 1 step from index 0 to 1, then 3 steps to the last index.

Note:

You can assume that you can always reach the last index.

 

AC code:

class Solution {
public:
    int jump(vector<int>& nums) {
        int start = 0, end = 0, len = nums.size();
        int step = 0, maxend = 0;
        while (end < len-1) {
            step++;
            maxend = end + 1;
            for (int i = start; i <= end; ++i) {
                if (i+nums[i] > len-1) return step;
                maxend = max(maxend, i+nums[i]);
            }
            start = end + 1;
            end = maxend;
        }
        return step;
    }
};

Runtime: 16 ms, faster than 22.35% of C++ online submissions for Jump Game II.

 

BFS:

class Solution {
public:
    int jump(vector<int>& nums) {
        int steps =1, i, curEnd = 0, newEnd=0, len = nums.size();
        if(len<=1) return 0;
        for(i=0; i<=curEnd; ++i)
        {
            newEnd = max(newEnd, i+nums[i]);
            if(newEnd>=(len-1)) return steps;
            if(i==curEnd)  // end of the current level, move to the next level
            {
                curEnd = newEnd;
                ++steps;
            }
        }
        return INT_MAX; // curEnd = newEnd, can not reach the end
    }
};

Runtime: 8 ms, faster than 98.58% of C++ online submissions for Jump Game II.