← Arrays

Find Missing Number

Easy
Python
def missing_number(nums):
    n=len(nums)
    return n*(n+1)//2-sum(nums)
Java
public int missingNumber(int[] nums){
    int n=nums.length,s=0;
    for(int x:nums) s+=x;
    return n*(n+1)/2-s;
}

Key Insight

Gauss formula. Python sum() eliminates loop.

Python → Java Differences

  • Python sum() vs Java loop
  • // vs /
  • Gauss formula identical
Python
def missing_number(nums):
    n=len(nums)
    return n*(n+1)//2-sum(nums)
Java
public int missingNumber(int[] nums){
    int n=nums.length,s=0;
    for(int x:nums) s+=x;
    return n*(n+1)/2-s;
}

Algorithm Steps

1. Expected - actual sum
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