https://oj.leetcode.com/problems/first-missing-positive/

http://blog.csdn.net/linhuanmars/article/details/20884585

public class Solution {
    public int firstMissingPositive(int[] A) {
        
        // 如果可以排序，排序后从1开始检查
        
        // 如果一个数为3，把它置换到第三个空格
        // i不动，继续置换，死循环！
        // 如果i位 的值 < 0,i++
        
        if (A == null || A.length == 0)
            return 1;
        
        int i = 0;
        while (i < A.length)
        {
            if (A[i] <= 0 || A[i] > A.length)
            {
                // Invalid number we don‘t care
                A[i] = -1;
                i++;
                continue;
            }
            
            if (A[i] == i + 1)
            {
                // Aready good
                i ++;
                continue;
            }
            
            // No need to swap
            // Avoid dead loop
            // 定义j方便coding
            int j = A[i] - 1;
            if (A[j] == A[i])
            {
                A[i] = -1;
                i ++;
                continue;
            }
            
            // swap A[i] and A[A[i] - 1]
            int t = A[i];
            A[i] = A[j];
            A[j] = t;
        }
        
        // Check first -1
        for (i = 0 ; i < A.length ; i ++)
        {
            if (A[i] < 0)
                return i + 1;
        }
        return A.length + 1;
    }
}

[LeetCode]41 First Missing Positive