LeetCode 45. Jump Game II

原题链接在这里: https://leetcode.com/problems/jump-game-ii/

题目：

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.

题解：

Have max initialized as 0. Up to max, keep updating reach range, when we hit max, we need to jump.

And assign the new range to max, if max is already beyong nums.length, return steps.

Note: Corner case [0], it should return 0. Thus at the beginning, if nums.length < 2, then return 0.

Time Complexity: O(n).

Space: O(1).

AC Java:

class Solution {
    public int jump(int[] nums) {
        if(nums == null || nums.length < 2){
            return 0;
        }
        
        int max = 0;
        int reach = 0;
        int res = 0;
        for(int i = 0; i<nums.length; i++){
            reach = Math.max(reach, i + nums[i]);
            if(i == max){
                res++;
                max = reach;
                if(max >= nums.length - 1){
                    return res;
                }
            }
        }
        
        return -1;
    }
}

类似Jump Game.