LeetCode #1365 — EASY

How Many Numbers Are Smaller Than the Current Number

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

The Problem

Problem Statement

Given the array nums, for each nums[i] find out how many numbers in the array are smaller than it. That is, for each nums[i] you have to count the number of valid j's such that j != i and nums[j] < nums[i].

Return the answer in an array.

Example 1:

Input: nums = [8,1,2,2,3]
Output: [4,0,1,1,3]
Explanation: 
For nums[0]=8 there exist four smaller numbers than it (1, 2, 2 and 3). 
For nums[1]=1 does not exist any smaller number than it.
For nums[2]=2 there exist one smaller number than it (1). 
For nums[3]=2 there exist one smaller number than it (1). 
For nums[4]=3 there exist three smaller numbers than it (1, 2 and 2).

Example 2:

Input: nums = [6,5,4,8]
Output: [2,1,0,3]

Example 3:

Input: nums = [7,7,7,7]
Output: [0,0,0,0]

Constraints:

2 <= nums.length <= 500
0 <= nums[i] <= 100

Step 01

Brute Force Baseline

Problem summary: Given the array nums, for each nums[i] find out how many numbers in the array are smaller than it. That is, for each nums[i] you have to count the number of valid j's such that j != i and nums[j] < nums[i]. Return the answer in an array.

Baseline thinking

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

Pattern signal: Array · Hash Map

Example 1

[8,1,2,2,3]

Example 2

[6,5,4,8]

Example 3

[7,7,7,7]

Core Insight

What unlocks the optimal approach

Brute force for each array element.
In order to improve the time complexity, we can sort the array and get the answer for each array element.

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 #1365: How Many Numbers Are Smaller Than the Current Number
class Solution {
    public int[] smallerNumbersThanCurrent(int[] nums) {
        int[] arr = nums.clone();
        Arrays.sort(arr);
        for (int i = 0; i < nums.length; ++i) {
            nums[i] = search(arr, nums[i]);
        }
        return nums;
    }

    private int search(int[] nums, int x) {
        int l = 0, r = nums.length;
        while (l < r) {
            int mid = (l + r) >> 1;
            if (nums[mid] >= x) {
                r = mid;
            } else {
                l = mid + 1;
            }
        }
        return l;
    }
}

// Accepted solution for LeetCode #1365: How Many Numbers Are Smaller Than the Current Number
func smallerNumbersThanCurrent(nums []int) (ans []int) {
	arr := make([]int, len(nums))
	copy(arr, nums)
	sort.Ints(arr)
	for i, x := range nums {
		nums[i] = sort.SearchInts(arr, x)
	}
	return nums
}

# Accepted solution for LeetCode #1365: How Many Numbers Are Smaller Than the Current Number
class Solution:
    def smallerNumbersThanCurrent(self, nums: List[int]) -> List[int]:
        arr = sorted(nums)
        return [bisect_left(arr, x) for x in nums]

// Accepted solution for LeetCode #1365: How Many Numbers Are Smaller Than the Current Number
struct Solution;

impl Solution {
    fn smaller_numbers_than_current(nums: Vec<i32>) -> Vec<i32> {
        let mut count = vec![0; 101];
        for &x in &nums {
            count[x as usize] += 1;
        }
        for i in 0..100 {
            count[i + 1] += count[i]
        }
        let mut res = vec![];
        for &x in &nums {
            let v = if x == 0 { 0 } else { count[(x - 1) as usize] };
            res.push(v)
        }
        res
    }
}

#[test]
fn test() {
    let nums = vec![8, 1, 2, 2, 3];
    let res = vec![4, 0, 1, 1, 3];
    assert_eq!(Solution::smaller_numbers_than_current(nums), res);
    let nums = vec![6, 5, 4, 8];
    let res = vec![2, 1, 0, 3];
    assert_eq!(Solution::smaller_numbers_than_current(nums), res);
    let nums = vec![7, 7, 7, 7];
    let res = vec![0, 0, 0, 0];
    assert_eq!(Solution::smaller_numbers_than_current(nums), res);
}

// Accepted solution for LeetCode #1365: How Many Numbers Are Smaller Than the Current Number
function smallerNumbersThanCurrent(nums: number[]): number[] {
    const search = (nums: number[], x: number) => {
        let l = 0,
            r = nums.length;
        while (l < r) {
            const mid = (l + r) >> 1;
            if (nums[mid] >= x) {
                r = mid;
            } else {
                l = mid + 1;
            }
        }
        return l;
    };
    const arr = nums.slice().sort((a, b) => a - b);
    for (let i = 0; i < nums.length; ++i) {
        nums[i] = search(arr, nums[i]);
    }
    return nums;
}

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 × log n)

Space

O(n)

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.

Mutating counts without cleanup

Wrong move: Zero-count keys stay in map and break distinct/count constraints.

Usually fails on: Window/map size checks are consistently off by one.

Fix: Delete keys when count reaches zero.