LeetCode #115 — HARD

Distinct Subsequences

Break down a hard problem into reliable checkpoints, edge-case handling, and complexity trade-offs.

The Problem

Problem Statement

Given two strings s and t, return the number of distinct subsequences of s which equals t.

The test cases are generated so that the answer fits on a 32-bit signed integer.

Example 1:

Input: s = "rabbbit", t = "rabbit"
Output: 3
Explanation:
As shown below, there are 3 ways you can generate "rabbit" from s.
rabbbit
rabbbit
rabbbit

Example 2:

Input: s = "babgbag", t = "bag"
Output: 5
Explanation:
As shown below, there are 5 ways you can generate "bag" from s.
babgbag
babgbag
babgbag
babgbag
babgbag

Constraints:

1 <= s.length, t.length <= 1000
s and t consist of English letters.

Step 01

Brute Force Baseline

Problem summary: Given two strings s and t, return the number of distinct subsequences of s which equals t. The test cases are generated so that the answer fits on a 32-bit signed integer.

Baseline thinking

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

Pattern signal: Dynamic Programming

Example 1

"rabbbit"
"rabbit"

Example 2

"babgbag"
"bag"

Core Insight

What unlocks the optimal approach

No official hints in dataset. Start from constraints and look for a monotonic or reusable state.

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

Largest constraint values

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 #115: Distinct Subsequences
class Solution {
    public int numDistinct(String s, String t) {
        int m = s.length(), n = t.length();
        int[][] f = new int[m + 1][n + 1];
        for (int i = 0; i < m + 1; ++i) {
            f[i][0] = 1;
        }
        for (int i = 1; i < m + 1; ++i) {
            for (int j = 1; j < n + 1; ++j) {
                f[i][j] = f[i - 1][j];
                if (s.charAt(i - 1) == t.charAt(j - 1)) {
                    f[i][j] += f[i - 1][j - 1];
                }
            }
        }
        return f[m][n];
    }
}

// Accepted solution for LeetCode #115: Distinct Subsequences
func numDistinct(s string, t string) int {
	m, n := len(s), len(t)
	f := make([][]int, m+1)
	for i := range f {
		f[i] = make([]int, n+1)
	}
	for i := 0; i <= m; i++ {
		f[i][0] = 1
	}
	for i := 1; i <= m; i++ {
		for j := 1; j <= n; j++ {
			f[i][j] = f[i-1][j]
			if s[i-1] == t[j-1] {
				f[i][j] += f[i-1][j-1]
			}
		}
	}
	return f[m][n]
}

# Accepted solution for LeetCode #115: Distinct Subsequences
class Solution:
    def numDistinct(self, s: str, t: str) -> int:
        m, n = len(s), len(t)
        f = [[0] * (n + 1) for _ in range(m + 1)]
        for i in range(m + 1):
            f[i][0] = 1
        for i, a in enumerate(s, 1):
            for j, b in enumerate(t, 1):
                f[i][j] = f[i - 1][j]
                if a == b:
                    f[i][j] += f[i - 1][j - 1]
        return f[m][n]

// Accepted solution for LeetCode #115: Distinct Subsequences
impl Solution {
    #[allow(dead_code)]
    pub fn num_distinct(s: String, t: String) -> i32 {
        let n = s.len();
        let m = t.len();
        let mut dp: Vec<Vec<u64>> = vec![vec![0; m + 1]; n + 1];

        // Initialize the dp vector
        for i in 0..=n {
            dp[i][0] = 1;
        }

        // Begin the actual dp process
        for i in 1..=n {
            for j in 1..=m {
                dp[i][j] = if s.as_bytes()[i - 1] == t.as_bytes()[j - 1] {
                    dp[i - 1][j] + dp[i - 1][j - 1]
                } else {
                    dp[i - 1][j]
                };
            }
        }

        dp[n][m] as i32
    }
}

// Accepted solution for LeetCode #115: Distinct Subsequences
function numDistinct(s: string, t: string): number {
    const m = s.length;
    const n = t.length;
    const f: number[][] = new Array(m + 1).fill(0).map(() => new Array(n + 1).fill(0));
    for (let i = 0; i <= m; ++i) {
        f[i][0] = 1;
    }
    for (let i = 1; i <= m; ++i) {
        for (let j = 1; j <= n; ++j) {
            f[i][j] = f[i - 1][j];
            if (s[i - 1] === t[j - 1]) {
                f[i][j] += f[i - 1][j - 1];
            }
        }
    }
    return f[m][n];
}

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(m × n)

Space

O(m × n)

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.

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.