LeetCode-in-Java

754. Reach a Number

Medium

You are standing at position 0 on an infinite number line. There is a destination at position target.

You can make some number of moves numMoves so that:

• On each move, you can either go left or right.
• During the ith move (starting from i == 1 to i == numMoves), you take i steps in the chosen direction.

Given the integer target, return the minimum number of moves required (i.e., the minimum numMoves) to reach the destination.

Example 1:

Input: target = 2

Output: 3

Explanation:

On the 1^st move, we step from 0 to 1 (1 step).

On the 2^nd move, we step from 1 to -1 (2 steps).

On the 3^rd move, we step from -1 to 2 (3 steps).

Example 2:

Input: target = 3

Output: 2

Explanation:

On the 1^st move, we step from 0 to 1 (1 step).

On the 2^nd move, we step from 1 to 3 (2 steps).

Constraints:

• -109 <= target <= 109
• target != 0

Solution

public class Solution {
    public int reachNumber(int target) {
        target = Math.abs(target);
        int val = (((int) Math.sqrt(1.0 + 8 * ((long) target))) - 1) / 2;
        int sum = (val * (val + 1)) / 2;
        while (sum < target || (sum - target) % 2 != 0) {
            val++;
            sum = sum + val;
        }
        return val;
    }
}

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