LeetCode | First Missing Positive

题目

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.

由于只能使用常数的额外空间，因此需要利用数组本身来保存所需信息。

需要注意的一点就是第一个缺失的正整数，最多就到N + 1。

代码

public class FirstMissingPositive {
	public int firstMissingPositive(int[] A) {
		if (A == null || A.length == 0) {
			return 1;
		}
		int N = A.length;
		for (int i = 0; i < N; ++i) {
			if (A[i] <= 0) {
				A[i] = N + 1;
			}
		}
		for (int i = 0; i < N; ++i) {
			if (Math.abs(A[i]) <= N) {
				int index = Math.abs(A[i]) - 1;
				A[index] = -Math.abs(A[index]);
			}
		}
		for (int i = 0; i < N; ++i) {
			if (A[i] > 0) {
				return i + 1;
			}
		}
		return N + 1;
	}
}

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LeetCode | First Missing Positive

原文：http://blog.csdn.net/perfect8886/article/details/22572577