0055. Jump Game

https://leetcode.com/problems/jump-game

Description

You are given an integer array nums. You are initially positioned at the array's first index, and each element in the array represents your maximum jump length at that position.

Return true if you can reach the last index, or false otherwise.

Example 1:

**Input:** nums = [2,3,1,1,4]
**Output:** true
**Explanation:** Jump 1 step from index 0 to 1, then 3 steps to the last index.

Example 2:

**Input:** nums = [3,2,1,0,4]
**Output:** false
**Explanation:** You will always arrive at index 3 no matter what. Its maximum jump length is 0, which makes it impossible to reach the last index.

Constraints:

• 1 <= nums.length <= 104

• 0 <= nums[i] <= 105

ac1

acepted, but ugly corner case is tedious

class Solution {
    public boolean canJump(int[] nums) {
        if (nums.length == 0) return false;
        if (nums.length == 1) return true;
        // start backwards
        // meet 0, raise zeroBlock
        // check current step can jump over zeroBlock, i + val > zeroBlock
        int zeroBlock = -1;
        for (int i = nums.length - 1 - 1; i >= 0; i--) {
            if (nums[i] == 0 && zeroBlock == -1) {
                zeroBlock = i;
                continue;
            }
            if (zeroBlock == -1) continue;
            if (nums[i] + i > zeroBlock) {
                zeroBlock = -1;
            }
        }

        return zeroBlock == -1;
    }
}

ac2

This one is concise and beautiful.

class Solution {
    public boolean canJump(int[] nums) {
        int reachable = nums.length - 1;
        for (int i = nums.length - 1; i >= 0; i--) {
            if (nums[i] + i >= reachable) reachable = i;
        }
        return reachable == 0;
    }
}