public int jump(int[] nums) {
    int n = nums.length;
    int ans = 0;
    int prevIdx = 0, lastIdx = 0;
    for (int i = 0; i < n; i++) {
        if (i > lastIdx) {
            return -1;
        }
        lastIdx = Math.max(lastIdx, i + nums[i]);
        if (i != n - 1 && i == prevIdx) {
            ans++;
            prevIdx = lastIdx;
        }
    }
    return ans;
}

55. Jump Game

public boolean canJump(int[] nums) {
    int n = nums.length;
    int lastIdx = 0;
    for (int i = 0; i < n; i++) {
        if (i > lastIdx) {
            return false;
        }
        lastIdx = Math.max(lastIdx, i + nums[i]);
    }
    return true;
}