45. Jump Game II

You are given a 0-indexed array of integers nums of length n. You are initially positioned at nums[0].

Each element nums[i] represents the maximum length of a forward jump from index i. In other words, if you are at nums[i], you can jump to any nums[i + j] where:

• 0 <= j <= nums[i] and
• i + j < n

Return the minimum number of jumps to reach nums[n - 1]. The test cases are generated such that you can reach nums[n - 1].

 

Example 1:

Input: nums = [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.

Example 2:

Input: nums = [2,3,0,1,4]
Output: 2

 

public int fun2(int[] nums) {
    if (nums == null || nums.length <= 1) {
        return 0;
    }
    int maxPosition = 0;       //记录从当前点能达到的最大位置
    int preMax = 0;      //记录前面能到达的最大点
    int step = 0;

    for (int i = 0; i < nums.length; i++) {
        if (i > preMax) {
            step++;
            preMax = maxPosition;
        }
        maxPosition = Math.max(maxPosition, i + nums[i]);
    }
    return step;
}