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LeetCode-121-Best-Time-to-Buy-and-Sell-Stock.java
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LeetCode-121-Best-Time-to-Buy-and-Sell-Stock.java
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/*
LeetCode: https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
LintCode: http://www.lintcode.com/problem/best-time-to-buy-and-sell-stock/
JiuZhang: http://www.jiuzhang.com/solutions/best-time-to-buy-and-sell-stock/
ProgramCreek: http://www.programcreek.com/2014/02/leetcode-best-time-to-buy-and-sell-stock-java/
Analysis:
DP
*/
class Solution {
// 1. Two Way Scan
/*
first array: left min view
second array: right max view
Time O(N)
Space O(N)
*/
// public int maxProfit(int[] prices) {
// if (prices == null || prices.length == 0) return 0;
// int n = prices.length;
// int[] left = new int[n];
// int leftMin = Integer.MAX_VALUE;
// for (int i = 0; i < n; i++) {
// leftMin = Math.min(leftMin, prices[i]);
// left[i] = leftMin;
// }
// int[] right = new int[n];
// int rightMax = Integer.MIN_VALUE;
// for (int i = n - 1; i >= 0; i--) {
// rightMax = Math.max(rightMax, prices[i]);
// right[i] = rightMax;
// }
// int max = Integer.MIN_VALUE;
// for (int i = 0; i < n; i++) {
// if (left[i] < right[i]) {
// max = Math.max(max, right[i] - left[i]);
// }
// }
// return max == Integer.MIN_VALUE ? 0 : max;
// }
// 2. DP (Pull)
/*
subproblem:
dp[i] - the highest prices by ith day
recurrence:
dp[i] = max(dp[i - 1], prices[i] - min);
init:
dp[0] = 0
ans:
dp[n - 1]
*/
// public int maxProfit(int[] prices) {
// if (prices == null || prices.length == 0) return 0;
// int n = prices.length;
// // subproblem
// int[] dp = new int[n];
// // init
// dp[0] = 0;
// int min = prices[0];
// // recurrence relation
// for (int i = 1; i < n; i++) {
// min = Math.min(min, prices[i]);
// dp[i] = Math.max(dp[i - 1], prices[i] - min);
// }
// // ans
// return dp[n - 1];
// }
// 3. Greedy
public int maxProfit(int[] prices) {
if (prices == null || prices.length == 0) return 0;
int n = prices.length;
int min = prices[0], maxProfit = 0;
for (int i = 1; i < n; i++) {
min = Math.min(min, prices[i]);
maxProfit = Math.max(maxProfit, prices[i] - min);
}
return maxProfit;
}
}