LeetCode刷题：41. First Missing Positive
原题链接：https://leetcode.com/problems/first-missing-positive/
Given an unsorted integer array, find the smallest missing positive integer.
Example 1:
Input: [1,2,0]
Output: 3
Example 2:
Input: [3,4,-1,1]
Output: 2
Example 3:
Input: [7,8,9,11,12]
Output: 1
Note:
Your algorithm should run in O(n) time and uses constant extra space.
算法设计
package com.bean.algorithmbasic;
/*
* 41. 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.
* */
public class FirstMissingPositive {
public static int findFirstMissingPositive1(int[] nums) {
int n = nums.length;
// 1. mark numbers (num < 0) and (num > n) with a special marker number (n+1)
// (we can ignore those because if all number are > n then we'll simply return 1)
for (int i = 0; i < n; i++) {
if (nums[i] <= 0 || nums[i] > n) {
nums[i] = n + 1;
}
}
// note: all number in the array are now positive, and on the range 1..n+1
// 2. mark each cell appearing in the array, by converting the index for that number to negative
for (int i = 0; i < n; i++) {
int num = Math.abs(nums[i]);
if (num > n) {
continue;
}
num--; // -1 for zero index based array (so the number 1 will be at pos 0)
if (nums[num] > 0) { // prevents double negative operations
nums[num] = -1 * nums[num];
}
}
// 3. find the first cell which isn't negative (doesn't appear in the array)
for (int i = 0; i < n; i++) {
if (nums[i] >= 0) {
return i + 1;
}
}
// 4. no positive numbers were found, which means the array contains all numbers 1..n
return n + 1;
}
public static int findFirstMissingPositive2(int[] target) {
if (target.length == 0 || target == null)
return 1;
// 把元素放入正确的位置，例如1放在target[0]，2放在target[1]...
for (int i = 0; i < target.length; i++) {
while (target[i] != i + 1) {
if (target[i] >= target.length || target[i] <= 0 || target[i] == target[target[i] - 1])
break;
int temp = target[i];
target[i] = target[temp - 1];
target[temp - 1] = temp;
}
}
for (int i = 0; i < target.length; i++) {
if (target[i] != i + 1)
return i + 1;
}
return target.length + 1;
}
public static int findFirstMissingPositive3(int[] nums) {
if(nums.length==0){
return 1;
}
int ans=1;
for(int i=0;i<nums.length;i++){
if(nums[i]==i+1){
continue;
}
while(nums[i]<=nums.length&&nums[i]>0&&nums[i]!=nums[nums[i]-1]){
int temp=nums[nums[i]-1];
nums[nums[i]-1]=nums[i];
nums[i]=temp;
}
}
int k=0;
for(k=0;k<nums.length;k++){
if(nums[k]!=k+1){
break;
}
}
return k+1;
}
public static void main(String[] args) {
// TODO Auto-generated method stub
int[] demo = new int[] { 3, 4, -1, 1 };
// int[] demo=new int[] {1,2,0};
int result = findFirstMissingPositive3(demo);
System.out.println("result= " + result);
}
}
程序运行结果：
result= 2