LeetCode #1773 — EASY

Count Items Matching a Rule

Build confidence with an intuition-first walkthrough focused on array fundamentals.

The Problem

Problem Statement

You are given an array items, where each items[i] = [type_i, color_i, name_i] describes the type, color, and name of the i^th item. You are also given a rule represented by two strings, ruleKey and ruleValue.

The i^th item is said to match the rule if one of the following is true:

ruleKey == "type" and ruleValue == type_i.
ruleKey == "color" and ruleValue == color_i.
ruleKey == "name" and ruleValue == name_i.

Return the number of items that match the given rule.

Example 1:

Input: items = [["phone","blue","pixel"],["computer","silver","lenovo"],["phone","gold","iphone"]], ruleKey = "color", ruleValue = "silver"
Output: 1
Explanation: There is only one item matching the given rule, which is ["computer","silver","lenovo"].

Example 2:

Input: items = [["phone","blue","pixel"],["computer","silver","phone"],["phone","gold","iphone"]], ruleKey = "type", ruleValue = "phone"
Output: 2
Explanation: There are only two items matching the given rule, which are ["phone","blue","pixel"] and ["phone","gold","iphone"]. Note that the item ["computer","silver","phone"] does not match.

Constraints:

1 <= items.length <= 10⁴
1 <= type_i.length, color_i.length, name_i.length, ruleValue.length <= 10
ruleKey is equal to either "type", "color", or "name".
All strings consist only of lowercase letters.

Step 01

Brute Force Baseline

Problem summary: You are given an array items, where each items[i] = [typei, colori, namei] describes the type, color, and name of the ith item. You are also given a rule represented by two strings, ruleKey and ruleValue. The ith item is said to match the rule if one of the following is true: ruleKey == "type" and ruleValue == typei. ruleKey == "color" and ruleValue == colori. ruleKey == "name" and ruleValue == namei. Return the number of items that match the given rule.

Baseline thinking

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

Pattern signal: Array

Example 1

[["phone","blue","pixel"],["computer","silver","lenovo"],["phone","gold","iphone"]]
"color"
"silver"

Example 2

[["phone","blue","pixel"],["computer","silver","phone"],["phone","gold","iphone"]]
"type"
"phone"

Step 02

Core Insight

What unlocks the optimal approach

Iterate on each item, and check if each one matches the rule according to the statement.

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 #1773: Count Items Matching a Rule
class Solution {
    public int countMatches(List<List<String>> items, String ruleKey, String ruleValue) {
        int i = ruleKey.charAt(0) == 't' ? 0 : (ruleKey.charAt(0) == 'c' ? 1 : 2);
        int ans = 0;
        for (var v : items) {
            if (v.get(i).equals(ruleValue)) {
                ++ans;
            }
        }
        return ans;
    }
}

// Accepted solution for LeetCode #1773: Count Items Matching a Rule
func countMatches(items [][]string, ruleKey string, ruleValue string) (ans int) {
	i := map[byte]int{'t': 0, 'c': 1, 'n': 2}[ruleKey[0]]
	for _, v := range items {
		if v[i] == ruleValue {
			ans++
		}
	}
	return
}

# Accepted solution for LeetCode #1773: Count Items Matching a Rule
class Solution:
    def countMatches(self, items: List[List[str]], ruleKey: str, ruleValue: str) -> int:
        i = 0 if ruleKey[0] == 't' else (1 if ruleKey[0] == 'c' else 2)
        return sum(v[i] == ruleValue for v in items)

// Accepted solution for LeetCode #1773: Count Items Matching a Rule
impl Solution {
    pub fn count_matches(items: Vec<Vec<String>>, rule_key: String, rule_value: String) -> i32 {
        let key = if rule_key == "type" {
            0
        } else if rule_key == "color" {
            1
        } else {
            2
        };
        items.iter().filter(|v| v[key] == rule_value).count() as i32
    }
}

// Accepted solution for LeetCode #1773: Count Items Matching a Rule
function countMatches(items: string[][], ruleKey: string, ruleValue: string): number {
    const key = ruleKey === 'type' ? 0 : ruleKey === 'color' ? 1 : 2;
    return items.reduce((r, v) => r + (v[key] === ruleValue ? 1 : 0), 0);
}

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)

Space

O(1)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED

O(n) time

O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.

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.