First Missing Positive Given an unsorted integer array, find the first missing positive integer. Fo

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.

Java代码：

public class Solution {   public int firstMissingPositive(int[] A) {    if (A == null || A.length == 0) {        return 1;    }    // Put the corresponding positive value in the index that equals to its value    int len = A.length;    for (int i = 0; i < len;) {        int num = A[i];        if (num > 0 && num < len && num != i && num != A[num]) {            A[i] = A[num];            A[num] = num;        } else {            ++i;        }    }    // Scan the array to find the first value that is not equal to its index, then it is the missing value    // Test small examples first    int missingValue = A[0] == len ? len+1 : len;    for (int i = 1; i < len; ++i) {        if (A[i] != i) {            missingValue = i;            break;        }    }    return missingValue;}}