41. First Missing Positive

The idea is simple: put the positive integer n (n > 0 and n <= array.length) at index n - 1. Then traverse the array, if nums[index] != index + 1, then we find the first miss positive number.

public int firstMissingPositive(int[] nums) {
    for(int i = 0; i < nums.length; i++) {
	while( nums[i] > 0 && nums[i] <= nums.length ) {
	    int temp = nums[nums[i] - 1];
	    if( temp == nums[i] ) // If nums[nums[i] - 1] equals nums[i], then we don't need to exchange them
		break;
	    nums[nums[i] - 1] = nums[i];
	    nums[i] = temp;
	}
    }
    for(int i = 0; i < nums.length; i++) 
	if( nums[i] != i + 1 )
	    return i + 1;
    return nums.length + 1;
}