in place swap 笔记

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.

尝试下用两根指针进行调换排序，但是处理返回结果太麻烦

简单的方式是扫描一遍数组，当有misplace的数的时候，把它换到它相应的正确位置上去

public class Solution {
    public int firstMissingPositive(int[] A) {
        if (A == null) {
            return 1;
        } 
        for (int i = 0; i < A.length; i++) {
            while (A[i] > 0 && A[i] <= A.length && A[i] != (i+1)) {//找到位置不正确的点和正确位置的点交换
                int tmp = A[A[i]-1];
                if (tmp == A[i]) {
                    break;
                }
                A[A[i]-1] = A[i];
                A[i] = tmp;
            }
        }

        for (int i = 0; i < A.length; i++) {
            if (A[i] != (i+1)) {
                return i + 1;
            }
        }
        return A.length + 1;
    }
    
}