First Missing Positive

Given an unsorted integer array, find the first missing positive integer.

For example, Given [1,2,0] return 3, and [3,4,-1,1] return 2.

Your algorithm should run in O(n) time and uses constant space.

Misunderstanding on the problem:

find the first missing positive, not the missing consecutive number !!!

i.e. 7 5 8 return 1 not 6

Solution:

zero based:

int firstMissingPositiveAnd0(int A[]) { int n = A.length; for (int i = 0; i < n; i++) { // when the ith element is not i while (A[i] != i) { // no need to swap when ith element is out of range [0,n] if (A[i] < 0 || A[i] >= n) break; //handle duplicate elements if(A[i]==A[A[i]]) break; // swap elements int temp = A[i]; A[i] = A[temp]; A[temp] = temp; } } for (int i = 0; i < n; i++) { if (A[i] != i) return i; } return n; }

one based:

public int firstMissingPositive(int[] A) { int n = A.length; for (int i = 0; i < n; i++) { while (A[i] != i + 1) { if (A[i] <= 0 || A[i] >= n) break; if(A[i]==A[A[i]-1]) break; int temp = A[i]; A[i] = A[temp - 1]; A[temp - 1] = temp; } } for (int i = 0; i < n; i++){ if (A[i] != i + 1){ return i + 1; } } return n + 1; }