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 jump2(int[] nums) {
    int length = nums.length;
    int end = 0; // 表示上一步到达的最远位置
    int maxPosition = 0; // 表示当前能到达的最远位置
    int steps = 0;
    for (int i = 0; i < length - 1; i++) {
        maxPosition = Math.max(maxPosition, i + nums[i]);
        if (i == end) { // 判断如果当前位置到达了上一步能到达的最远位置，说明需要再跳一次了，并且步数steps自增1
            end = maxPosition;
            steps++;
        }
    }
    return steps;
}