[Easy] 455. Assign Cookies

Problem Statement

Assume you are an awesome parent and want to give your children some cookies. But, you should give each child at most one cookie.

Each child i has a greed factor g[i], which is the minimum size of a cookie that the child will be content with; and each cookie j has a size s[j]. If s[j] >= g[i], we can assign the cookie j to the child i, and the child i will be content. Your goal is to maximize the number of your content children and output the maximum number.

Examples

Example 1:

Input: g = [1,2,3], s = [1,1]
Output: 1
Explanation: You have 3 children and 2 cookies. The greed factors of 3 children are 1, 2, 3. 
And even though you have 2 cookies, since their size is both 1, you could only make the child whose greed factor is 1 content.
You need to output 1.

Example 2:

Input: g = [1,2], s = [1,2,3]
Output: 2
Explanation: You have 2 children and 3 cookies. The greed factors of 2 children are 1, 2. 
You have 3 cookies and their sizes are big enough to gratify all of the children, 
You need to output 2.

Constraints

  • 1 <= g.length <= 3 * 10^4
  • 0 <= s.length <= 3 * 10^4
  • 1 <= g[i], s[j] <= 2^31 - 1

Clarification Questions

Before diving into the solution, here are 5 important clarifications and assumptions to discuss during an interview:

  1. Assignment rule: Can one cookie satisfy multiple children? (Assumption: No - one cookie can only satisfy one child)

  2. Cookie size requirement: What size cookie is needed for a child? (Assumption: Cookie size must be >= child’s greed factor)

  3. Optimization goal: What are we optimizing for? (Assumption: Maximize the number of satisfied children - greedy approach)

  4. Cookie reuse: Can we use the same cookie multiple times? (Assumption: No - each cookie can only be used once)

  5. Ordering: Does the order of children or cookies matter? (Assumption: No - we can assign in any order, but sorting helps optimize)

Interview Deduction Process (10 minutes)

Step 1: Brute-Force Approach (2 minutes)

Initial Thought: “I need to assign cookies to children. Let me try all possible assignments.”

Naive Solution: Try all possible cookie-to-child assignments, count valid ones. Use nested loops or backtracking.

Complexity: O(n × m) or exponential time, O(1) space

Issues:

  • Very inefficient for large inputs
  • Doesn’t leverage greedy property
  • Overcomplicated for this problem

Step 2: Semi-Optimized Approach (3 minutes)

Insight: “I should match smallest cookie to smallest child first. This is a greedy approach.”

Improved Solution: Sort both arrays. Use two pointers to match smallest available cookie to smallest unsatisfied child greed factor.

Complexity: O(n log n + m log m) time, O(1) space

Improvements:

  • Greedy matching is optimal
  • Sorting enables efficient matching
  • Two-pointer technique is clean
  • Handles all cases correctly

Step 3: Optimized Solution (5 minutes)

Final Optimization: “The greedy two-pointer approach is already optimal. Sorting is necessary for correctness.”

Best Solution: Sort both arrays, use greedy matching with two pointers. This maximizes number of satisfied children.

Key Realizations:

  1. Greedy approach is optimal for this problem
  2. Sorting enables efficient O(n + m) matching after sort
  3. Two-pointer technique is elegant
  4. O(n log n) sorting dominates time complexity

Solution Approach

This is a classic greedy algorithm problem. The key insight is to use a greedy strategy: assign the smallest cookie that satisfies each child’s greed factor, starting with the child with the smallest greed factor.

Key Insights:

  1. Sorting: Sort both arrays to process in order
  2. Greedy Choice: Always assign the smallest cookie that satisfies a child
  3. Two Pointers: Use two pointers to match children with cookies efficiently
  4. Optimal: This greedy approach maximizes the number of content children

Algorithm:

  1. Sort: Sort g (children’s greed factors) and s (cookie sizes)
  2. Match: Use two pointers to match smallest cookie to smallest child
  3. Count: Count successful assignments

Solution

Solution 1: Two Pointers with Conditional Advancement

class Solution:
    def findContentChildren(self, g, s):
        g.sort()
        s.sort()
        count = 0
        i = 0
        j = 0
        while i < len(g) and j < len(s):
            if s[j] >= g[i]:
                count += 1
                i += 1
                j += 1
                return count

class Solution:
    def findContentChildren(self, g, s):
        g.sort()
        s.sort()
        num_of_children, num_of_cookies = len(g), len(s)
        count = 0
        j = 0
        for i in range(num_of_children):
            while j < num_of_cookies and g[i] > s[j]:
                j += 1
            if j < num_of_cookies:
                count += 1
                j += 1
            else:
                break
        return count

Algorithm Explanation:

Solution 1:

  1. Sort Arrays (Lines 4-5):
    • Sort g in ascending order (children’s greed factors)
    • Sort s in ascending order (cookie sizes)
  2. Two Pointers Matching (Lines 6-14):
    • Initialize: i for children, j for cookies, count for assignments
    • While both arrays have elements:
      • If cookie satisfies child: s[j] >= g[i]
        • Increment count
        • Move to next child (i++)
        • Move to next cookie (j++)
      • Else: Cookie too small, skip it (j++)

Solution 2:

  1. Sort Arrays (Lines 4-5): Same as Solution 1

  2. For Each Child (Lines 7-13):

    • Skip small cookies: Use while loop to skip cookies smaller than child’s greed
    • Check bounds: Ensure j < numOfCookies after skipping
    • Assign if valid: If cookie found, increment count
    • Advance both: Move to next child and cookie

Example Walkthrough:

Example 1: g = [1,2,3], s = [1,1]

After sorting:

g = [1, 2, 3]
s = [1, 1]

Solution 1 Execution:

Initial: i=0, j=0, count=0

Step 1: s[0]=1 >= g[0]=1 ✓
  count=1, i=1, j=1

Step 2: s[1]=1 < g[1]=2 ✗
  j=2 (out of bounds)

Result: count=1

Solution 2 Execution:

Initial: i=0, j=0, count=0

i=0: g[0]=1
  while: j=0, s[0]=1 >= 1, no skip
  j=0 < 2 ✓, count=1
  i=1, j=1

i=1: g[1]=2
  while: j=1, s[1]=1 < 2, j=2 (out of bounds)
  j=2 >= 2 ✗, no assignment

Result: count=1

Example 2: g = [1,2], s = [1,2,3]

After sorting:

g = [1, 2]
s = [1, 2, 3]

Solution 1 Execution:

Initial: i=0, j=0, count=0

Step 1: s[0]=1 >= g[0]=1 ✓
  count=1, i=1, j=1

Step 2: s[1]=2 >= g[1]=2 ✓
  count=2, i=2 (out of bounds)

Result: count=2

Solution 2 Execution:

Initial: i=0, j=0, count=0

i=0: g[0]=1
  while: j=0, s[0]=1 >= 1, no skip
  j=0 < 3 ✓, count=1
  i=1, j=1

i=1: g[1]=2
  while: j=1, s[1]=2 >= 2, no skip
  j=1 < 3 ✓, count=2
  i=2 (out of bounds)

Result: count=2

Algorithm Breakdown

Why Greedy Works

The greedy strategy is optimal because:

  1. Smallest First: Assign smallest cookie to smallest child maximizes remaining options
  2. No Regret: If we skip a cookie for a child, we can’t use it later (each child gets at most one)
  3. Optimal Substructure: After assigning a cookie, the subproblem (remaining children and cookies) is independent

Why Sorting is Necessary

Without sorting, we might:

  • Assign a large cookie to a child with small greed factor
  • Miss opportunities to satisfy more children
  • Need to check all combinations (O(n²))

With sorting:

  • Process in order (smallest to largest)
  • Greedy choice is optimal
  • Linear time matching

Comparison of Solutions

Solution 1:

  • Advancement: Cookie pointer advances in both branches
  • Logic: Clear if-else structure
  • Efficiency: Both pointers advance optimally

Solution 2:

  • Advancement: Cookie pointer skips multiple small cookies at once
  • Logic: Uses while loop to skip cookies
  • Efficiency: Same time complexity, slightly different style

Both solutions have the same time complexity and produce the same result.

Time & Space Complexity

  • Time Complexity:
    • Sorting: O(g log g + s log s) where g = g , s = s
    • Matching: O(g + s) - each element visited at most once
    • Total: O(g log g + s log s)
  • Space Complexity: O(1) - only using a few variables (excluding input arrays)

Key Points

  1. Greedy Algorithm: Always make locally optimal choice
  2. Sorting: Essential for greedy approach to work
  3. Two Pointers: Efficient matching technique
  4. Optimal: Greedy strategy maximizes number of content children
  5. Simple: Straightforward implementation

Alternative Approaches

Approach 1: Greedy with Sorting (Current Solutions)

  • Time: O(g log g + s log s)
  • Space: O(1)
  • Best for: General case, optimal solution

Approach 2: Brute Force (Check All Combinations)

  • Time: O(g × s)
  • Space: O(1)
  • Use when: Arrays are very small (not practical)

Approach 3: Binary Search (For Each Child)

  • Time: O(g log s)
  • Space: O(1)
  • Use when: Cookies array is much larger than children array

Edge Cases

  1. No cookies: s = [] → return 0
  2. No children: g = [] → return 0
  3. All cookies too small: g = [5,6,7], s = [1,2,3] → return 0
  4. All children satisfied: g = [1,2], s = [3,4,5] → return 2
  5. Single child, single cookie: g = [1], s = [1] → return 1

Tags

Array, Greedy, Sorting, Two Pointers, Easy