[Leetcode] 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.

 

方法比较巧，先把所有元素的值-1，重新组织元素使A[i] = i，这样只要找到第一个A[i] != i 的 i。注意要用while循环，for循环的continue是从下一次循环开始的。

 

class Solution {
public:
    int firstMissingPositive(int A[], int n) {
        int tmp;
        for (int i = 0; i < n; ++i) {
            A[i]--;
        }
        int i = 0;
        while (i < n) {
            if (A[i] != i && A[i] >= 0 && A[i] < n && A[i] != A[A[i]]) {
                tmp = A[i];
                A[i] = A[A[i]];
                A[tmp] = tmp;
            } else {
                i++;
            }
        }
        for (int i = 0; i < n; ++i) {
            if (A[i] != i) {
                return i + 1;
            }
        }
        return n + 1;
    }
};