LeetCode #213 — MEDIUM

House Robber II

Move from brute-force thinking to an efficient approach using array strategy.

The Problem

Problem Statement

You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed. All houses at this place are arranged in a circle. That means the first house is the neighbor of the last one. Meanwhile, adjacent houses have a security system connected, and it will automatically contact the police if two adjacent houses were broken into on the same night.

Given an integer array nums representing the amount of money of each house, return the maximum amount of money you can rob tonight without alerting the police.

Example 1:

Input: nums = [2,3,2]
Output: 3
Explanation: You cannot rob house 1 (money = 2) and then rob house 3 (money = 2), because they are adjacent houses.

Example 2:

Input: nums = [1,2,3,1]
Output: 4
Explanation: Rob house 1 (money = 1) and then rob house 3 (money = 3).
Total amount you can rob = 1 + 3 = 4.

Example 3:

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

Constraints:

1 <= nums.length <= 100
0 <= nums[i] <= 1000

Step 01

Brute Force Baseline

Problem summary: You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed. All houses at this place are arranged in a circle. That means the first house is the neighbor of the last one. Meanwhile, adjacent houses have a security system connected, and it will automatically contact the police if two adjacent houses were broken into on the same night. Given an integer array nums representing the amount of money of each house, return the maximum amount of money you can rob tonight without alerting the police.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: Array · Dynamic Programming

Example 1

[2,3,2]

Example 2

[1,2,3,1]

Example 3

[1,2,3]

Core Insight

What unlocks the optimal approach

Since House[1] and House[n] are adjacent, they cannot be robbed together. Therefore, the problem becomes to rob either House[1]-House[n-1] or House[2]-House[n], depending on which choice offers more money. Now the problem has degenerated to the <a href ="https://leetcode.com/problems/house-robber/description/">House Robber</a>, which is already been solved.

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #213: House Robber II
class Solution {
    public int rob(int[] nums) {
        int n = nums.length;
        if (n == 1) {
            return nums[0];
        }
        return Math.max(rob(nums, 0, n - 2), rob(nums, 1, n - 1));
    }

    private int rob(int[] nums, int l, int r) {
        int f = 0, g = 0;
        for (; l <= r; ++l) {
            int ff = Math.max(f, g);
            g = f + nums[l];
            f = ff;
        }
        return Math.max(f, g);
    }
}

// Accepted solution for LeetCode #213: House Robber II
func rob(nums []int) int {
	n := len(nums)
	if n == 1 {
		return nums[0]
	}
	return max(robRange(nums, 0, n-2), robRange(nums, 1, n-1))
}

func robRange(nums []int, l, r int) int {
	f, g := 0, 0
	for _, x := range nums[l : r+1] {
		f, g = max(f, g), f+x
	}
	return max(f, g)
}

# Accepted solution for LeetCode #213: House Robber II
class Solution:
    def rob(self, nums: List[int]) -> int:
        def _rob(nums):
            f = g = 0
            for x in nums:
                f, g = max(f, g), f + x
            return max(f, g)

        if len(nums) == 1:
            return nums[0]
        return max(_rob(nums[1:]), _rob(nums[:-1]))

// Accepted solution for LeetCode #213: House Robber II
impl Solution {
    pub fn rob(nums: Vec<i32>) -> i32 {
        let n = nums.len();
        if n == 1 {
            return nums[0];
        }
        let rob_range = |l, r| {
            let mut f = [0, 0];
            for i in l..r {
                f = [f[0].max(f[1]), f[0] + nums[i]];
            }
            f[0].max(f[1])
        };
        rob_range(0, n - 1).max(rob_range(1, n))
    }
}

// Accepted solution for LeetCode #213: House Robber II
function rob(nums: number[]): number {
    const n = nums.length;
    if (n === 1) {
        return nums[0];
    }
    const robRange = (l: number, r: number): number => {
        let [f, g] = [0, 0];
        for (; l <= r; ++l) {
            [f, g] = [Math.max(f, g), f + nums[l]];
        }
        return Math.max(f, g);
    };
    return Math.max(robRange(0, n - 2), robRange(1, n - 1));
}

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n × m)

Space

O(n × m)

Approach Breakdown

RECURSIVE

O(2ⁿ) time

O(n) space

Pure recursion explores every possible choice at each step. With two choices per state (take or skip), the decision tree has 2ⁿ leaves. The recursion stack uses O(n) space. Many subproblems are recomputed exponentially many times.

DYNAMIC PROGRAMMING

O(n × m) time

O(n × m) space

Each cell in the DP table is computed exactly once from previously solved subproblems. The table dimensions determine both time and space. Look for the state variables — each unique combination of state values is one cell. Often a rolling array can reduce space by one dimension.

Shortcut: Count your DP state dimensions → that’s your time. Can you drop one? That’s your space optimization.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.

State misses one required dimension

Wrong move: An incomplete state merges distinct subproblems and caches incorrect answers.

Usually fails on: Correctness breaks on cases that differ only in hidden state.

Fix: Define state so each unique subproblem maps to one DP cell.