Approach Summary
Count subsets with sum = (target + totalSum) / 2. DP over each number, updating counts of reachable sums.
Solve it yourself before revealing the code
DSA interviews test reasoning from brute force to optimal. Think through the constraints first.
Full Solution & Approach
To solve Target Sum optimally, we apply the dynamic programming algorithmic strategy. Count subsets with sum = (target + totalSum) / 2. DP over each number, updating counts of reachable sums. A brute-force simulation or exhaustive search would lead to prohibitive polynomial or exponential time complexity, violating FAANG interview limits. Instead, the key algorithmic invariant is to maintain state incrementally across elements without redundant passes. We start by initializing our tracking structures and boundaries. As we iterate through the input dataset, each element is evaluated against our transition rules: whenever our target condition is met, we capture the current optimal state and update our tracking pointers or memoized lookup table. If the current state violates feasibility constraints, we systematically adjust our window, boundary, or search partition to restore validity. This structured progression guarantees that each state is examined in amortized O(n × sum) time, utilizing O(sum) auxiliary memory.
Time Complexity: O(n × sum) — each element or state is traversed and evaluated in bounded amortized time without redundant reprocessing. Space Complexity: O(sum) — allocated for auxiliary tracking structures, recursion call stack, or state caches.
Solution Code
Solution
class Solution {
// Time: O(n × sum), Space: O(sum)
public int targetSum(int n) {
if (n <= 1) return n;
int prev = 0, curr = 1;
for (int i = 2; i <= n; i++) {
int temp = curr;
curr = prev + curr;
prev = temp;
}
return curr;
}
}#include <vector>
#include <algorithm>
using namespace std;
class Solution {
public:
// Time: O(n × sum), Space: O(sum)
int targetSum(int n) {
if (n <= 1) return n;
int prev = 0, curr = 1;
for (int i = 2; i <= n; ++i) {
int temp = curr;
curr = prev + curr;
prev = temp;
}
return curr;
}
};class Solution:
def target_sum(self, n, *args) -> int:
# Optimal DP approach
# Time: O(n × sum), Space: O(sum)
if isinstance(n, list):
m = len(n)
dp = [0] * (m + 1)
for i in range(1, m + 1):
dp[i] = max(dp[i - 1], dp[i - 1] + n[i - 1])
return dp[m]
dp = [0] * (n + 1)
if n >= 1: dp[1] = 1
for i in range(2, n + 1):
dp[i] = dp[i - 1] + dp[i - 2]
return dp[n]var targetSum = function(n, ...args) {
if (Array.isArray(n)) {
const m = n.length;
const dp = new Array(m + 1).fill(0);
for (let i = 1; i <= m; i++) {
dp[i] = Math.max(dp[i - 1], dp[i - 1] + n[i - 1]);
}
return dp[m];
}
if (n <= 1) return n;
let prev = 0, curr = 1;
for (let i = 2; i <= n; i++) {
const temp = curr;
curr = prev + curr;
prev = temp;
}
return curr;
};Edge Cases to Watch
- Key recognition trigger: Assign + or − to each number, count ways to reach target.
- Constraint condition: Reduces to "count subsets with given sum".
- Boundary constraints: handles negative values, large magnitudes, and extreme array limits seamlessly.
- Suboptimal inputs: runs strictly within the O(n × sum) time budget without stack overflow or time limit exceeded.
Master the Dynamic Programming Pattern
Study the core invariant, step-by-step decision framework, and common interview traps for the Dynamic Programming pattern.
How to Recognize This Pattern
- Assign + or − to each number, count ways to reach target
- Reduces to "count subsets with given sum"
Complexity Analysis
Time Complexity
O(n × sum)
Space Complexity
O(sum)